OCR Text |
Show 62 MR. O. THOMAS ON THE [Jail. 3, developed, and looking quite as if that species always possessed them. Again, a specimen of H. gracilis has an additional lower premolar standing side by side with the third normal one ; but this differs from the last instance in being more obviously an abnormality. The type of Rhinogale melleri, as mentioned below (p. 81), has one more upper premolar on each side than is found in any other species of the group; but as, so far as I know, this typical specimen is unique, I cannot say whether the dental formula possessed by it is constant or not. On the other hand, a deficiency in the full number of teeth is very common, all the species with normally four upper premolars often losing the first one, which when present is always much smaller than any of the others. It has just been mentioned that the dimensions of the teeth are very constant; and so much is this the case that I have found that the comparative measurements of the fourth upper premolar and second molar afford very good specific characters; for in those species in which the teeth are more or less round and suitable for grinding rather than cutting, the second molar is of considerable size and the fourth premolar is but little larger, while, on the other hand, when the teeth are slender, angular, and with cutting-edges, as in H. ichneumon, the last, molar is very small and rudimentary and the last premolar is large and has a well-developed anterior internal process. As au instance of these differences, I may mention the cases of H. albicauda and H. ichneumon, in the first of which the last molar is from 71 to 84 per cent, of the fourth premolar, while in the latter these percentages range only from 42 to 45. The measurements upon which these percentages are founded are those of the greatest diameter of the tooth-in the fourth premolar from the posterior point of the tooth to the anterior edge of the internal lobe, and in the last molar simply the greatest transverse diameter of the tooth. The percentages obtained by these measurements are called in the synopses of the species the " dental percentages;" and a table is given at the end of each genus showing the exact dimensions, with the resulting percentages, of the teeth of its members. Synopsis of the Genera. A. With a naked groove from the muzzle to the upper lip. I. Toes 5-5. a. Premolars j (if only <J in either jaw, a diastema always present) I. HEKFESTES. //. Premolars -jf- (no diastema in front of 1st premolar). Hind soles naked H. HELOGALE. II. Toes 4-4. c. Premolars :{-• Hind soles hairy III. BDEOGALE. III. Toes 5-4. d. Premolars j. Hind soles hairy , IV. CYNICTIS. |