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In every species, males surpass females in average size. Nevertheless, the largest animals are usually females. How this paradoxal fact is to be accounted for, I am not sure, but it may be that the animals grow as long as they live and that females have more chances to survive longer since the care of the young keeps them closer to shelter.

Insular populations are usually darker or richer in color than corresponding continental populations. On a small island, uniformity of environment and inbreeding may be responsible for an accumulation of characters for richness of color.

Pelage

Species with narrow aristiforms have a rather soft and flexible pelage, while those with wide aristiforms have harsh, spiny pelage. The aristiforms vary in width from 0.45 to 1.3 mm, depending upon the species or subspecies.

The setiforms are narrow and flattened but are without pronouncedly raised margins. The setiforms are usually bicolored on the dorsal and lateral surfaces of the animals, with a subapical zone of some reddish-brown color, like Ochraceous-Orange or Ochraceous-Buff. They are whitish or gray on the basal parts and gradually blacken toward the tip, but have a reddish subapical zone. Common exceptions to this pattern are setiforms without subapical zones; these appear on the dorsal surface among setiforms which are normal in possessing distinctive subapical zones. Also there are setiforms without blackened tips on the lateral surfaces. Due to their relative abundance and subapical color, these setiforms are responsible for the dominant color on the upper parts. Like the aristiforms, they are longer and wider in the middorsal region of the animal and are gradually less developed on the remainder of the upper parts. Actually there is more than one type of setiform in the hair constellation; they vary in length, width and color. Attention was not given, however, to every type of setiform.

The ventral surface of the body and the inner sides of the legs are uniformly covered by short setiforms, thinner and more sparsely distributed on the inner side of the legs. These setiforms are usually uniformly white in color or, sometimes, the distal parts are buff or more richly colored.

Vibrissiforms are scattered on the dorsal and lateral surfaces of the body, and in penicillate arrangements on the head. They are longer than the pelage proper, have a nearly circular cross-section and are blackish in color.

Skull

Linear and spatulate shape of the humular process of the pterygoid constituted specific characters for Thomas, but there is so much individual variation in the shape of this process in almost every population that it has not been used in the present account.

Incisive Foramen

The shape and dimensions of the incisive foramen long have been recognized as providing specific characters. Large size of the foramen is probably correlated with the requirement for a large amount of moisture reaching Jacobson's organ in the nasopalatine space; the moistening of the sensory epithelium is certainly involved. There seems to be a certain correlation between small size of the incisive foramen and high degree of humidity in the environment. Shapes and dimensions of the foramen appear as simple or multiple biotypes and provide characters which can be employed to differentiate subspecies, species and even subgenera. Usually a character, say a general shape, occurs in nearly all populations of a given subspecies but the particular shape seems to be more closely correlated with ecological conditions, especially humidity. Animals which live far away from large rivers usually have larger foramina than animals which live close to rivers.

Both the premaxilla and the maxilla develop processes which form a sheath for the vomer. This vomerine sheath forms a bridge which longitudinally crosses the incisive foramen; the structure of this bridge varies widely. Sometimes the maxillary part is not developed and the sheath is incomplete posteriorly; sometimes this maxillary part is very slender and merely touches the premaxillary part. The premaxillary part, however, is always well developed.

Teeth

Considered by itself the variation in the tooth pattern can lead to erroneous conclusions as to differentiation of species, because the number of folds on the occlusal face of a tooth and the depth of certain folds may be subject to great individual variation as shown by examination of more than one large series of specimens of the same kind, age and sex from a single locality. Also there are geographic gradients or clines, in number of folds. Nevertheless the variation in number of folds, when measured at sufficient intervals along a cline, may provide quantitative characters useful in differentiating subspecies.

Adjacent counterfolds may appear to be coalesced in many instances. Coalescence is more likely to be seen in species where a wider variation in the number of the counterfolds is involved and it appears as a gradient in the reduction of the number of counterfolds.

Of great importance, as a general feature of molariform teeth, is the relative size as related to the geographical distribution, showing, again, a natural division in the genus. In all forms of southeastern Brazil the premolars are larger than the first molars, the first molars are larger than the second molars, and the second molars are larger than the third molars. The forms from central and northern Bahia, Brazil, have the molariform teeth more or less the same size. The forms from the remaining part of the area occupied by the genus have premolars smaller than the first molars, the first molars smaller than the second molars, but the second molars larger than third ones.

HABITS

Sometimes the emergence from the nest is followed by a long yawning and stretching ceremony. The animal spreads the fore and hind legs widely apart, while the back is curved down and the head and tail turn upward. Then one of the hind legs is stretched backward and, at the same time, the mouth is opened widely and the tail is moved in an undulatory fashion. The operation may be repeated using the other legs, or not.

CHANGES WITH AGE

The weights and measurements represent averages of specimens of the different ages.

ARTIFICIAL KEY TO THE SUBGENERA AND SPECIES

Tail 90 per cent or more of head and body; aristiforms evident on outer thighs and rump; skull with no ridges across parietals; size of upper cheekteeth decreasing from P4 to M3; main fold large. subgenus TRINOMYS, 5

Between 13 and 23 January, 1934, A. M. Olalla collected 10 adult females, 6 of which contained embryos. Three of the females had 2 embryos each, two had 3 embryos each and one had only 1 embryo. At this same time and place only ten per cent of specimens obtained were not fully adult.

E. Snethlage collected one specimen in a garden . However, according to the personnel of the Brazilian Health Service, the animals are strictly forest dwellers although they do make excursions into more open places.

In discussing the type locality of the species, Thomas states: "We know that its donor did obtain a number of specimens from Rio Janeiro, and the skull agrees so closely with those of two examples from Itatiaia, near to the Rio-Minas frontier, collected and presented by Prof. J. P. Hill, that I have no hesitation in referring the latter to G?nther's species."

The forest where the animals were captured has a high percentage of deciduous trees in spite of the heavy rainfall in this region. All of the animals were trapped near water. Young were captured from January to May. Most animals have a conspicuous Cinnamon patch on the nuchal region.

Proechimys setosus

INCERTA SEDIS

CONCLUSIONS

TABLE OF MEASUREMENTS

Key:

A Length of head and body B Length of tail C Length of hind-foot D Length of ear from notch E Greatest length of skull F Condylo-incisive length G Zygomatic breadth H Length of nasals I Interorbital constriction J Palatilar length K Crown length of cheekteeth

LITERATURE CITED

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BERRY, E. W. 1942. Mesozoic and Cenozoic plants of South America, Central America and the Antilles. Proc. Eighth Amer. Sci. Congress, 4:365-373.

DESMAREST, A. G. 1817. Nouveau Dictionaire d'Histoire Naturelle, ed. 2, 10:59.

ELLERMAN, J. R. 1940. The families and genera of living Rodents. Vol. 1, Rodents other than Muridae, pp. xxvi + 689, 189 text figs. British Museum . June 8, 1940.

LICHTENSTEIN, M. H. C.

LUND, P. W. 1841. Blik paa Brasiliens Dyreverden f?r sidste Jordomvaeltning, af Dr. Lund. Kongelige Danske Videnskabernes Selskabs naturvidenska-belige og mathematiske. Fortsaettelse af Pattedyrene. Kj?benhavn, 2:217-272, pls. 14-24.

OSGOOD, WILFRED H. 1944. Nine new South American rodents. Zool. Ser. Field Mus. Nat. Hist., 29:191-204, July 12, 1944.

RIBEIRO, ALIPIO DE MIRANDA 1914. Historia Natural. Zoologia. Mammiferos. Commiss?o de Linhas Telegraficas, Estrategicas de Matto Grosso ao Amazonas. Annexo no. 5, Rio de Janeiro, pp. 1-49 + 1-3, pls. 25, May, 1914.

RIDGWAY, ROBERT 1912. Color standards and color nomenclature, iv + 44 pp., 53 pls. Published by the author, Washington, D. C.

TATE, G. H. H. 1935. The taxonomy of the genera of Neotropical hystricoid rodents. Bull. Amer. Mus. Nat. Hist., 68:295-447, June 12, 1935.

THOMAS, OLDFIELD 1900. Descriptions of new rodents from western South America. Ann. and Mag. Nat. Hist., 6:294-302, September, 1900.

WAGNER, ANDREAS 1843. Beschreibung einiger neuer Nager, welche auf der Reise des Herrn Hofrats v. Schubert gesammelt wurden, mit Bezugnahme auf einige andere verwandte Formen. Abhandl. Akad. Wiss. math.-phys. Cl., 3:173-216, pl. 2.

WAGNER, J. A. 1844 . In Schreber's Die S?ugethiere ..., 7 pts. including text and pls., colored .

WINGE, HERLUF 1888. Jordfundne og nulevende Gnavere fra Lagoa Santa, Minas Gerais, Brasilien. E Museo Lundii Afhandlinger, 1:1-178, pls. 1-8.

Transcriber's notes:

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