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Show 76 ZOOLOGY OF TilE VOYAGE OF TIJE BEAGLE. same Plate- recede still farther from tl1e genus JIIus, and. approach more nearly (as regards the dentition) to the A1·vicolidm. Among the species here described 1 may mention as examples, M. griseojlavus, M. zantlwpygns, and M. Da1·winii; -see the molar teeth ii()'ured in Plate 34. figs. 15, W, and 17, -and amon()' the North American species, those constituting the genus Neotoma. The Iauer make by far tbe nearest approach to the A1·vicolidm of any which have yet come tmder my ob ervation, not only in the dentition, but in the form of the skull and the large s ize of the coronoid process of the lower jaw; there is, nevertheless, a tolerably well marked line of d.istinction between the crania of the Arvicolidce and Neoloma. The skulls of the animals belonging to the genera Caslo1·, Ondat1'U, Arvi-coln, Spala.r:, and Geomys, which constitute the principal groups of the family A1"1.:ico1idce, when compared with those of the family Mw·idm, present, among others, the following distinctive characters. The temporal fossm are always much contracted posteriorly, by the great anterior and lateral development of the temporal bones ; the plane of the intermolat · portion of the palate is below the level of the anterior portion ; the coronoid process of the lower jaw is very large, the articular portion of the COJH.lyloiu process is proportionately broad; the descending ramus, or posterior coronoid process, is so situated that its upper portion terminates considerably above the level of the crowns of the molars ; this same process is generally * directed. outwards from the plane of the horizontal ramus. Tl1e incisor teeth of the A1·vicolidce differ from those of the ~fu1·idm in being proportionately broader and less deep from front to back-they are not laterally compressed as in Mus. The molar teeth are rootless,t and the folds of enamel are the same throughout the whole length of the tooth; whereas in Mus they enter less and less deeply into the body of the tooth as we recede from the crown, and toward.s the base of the visible portion (the tooth being in its socket) the indentations of the enamel are obliterated. Now in the species of Ilespet·omys, the molar teeth arc always rooted, and in the form of the skull and the lower jaw they agree with the Muridm, and do not '" I am acquainted with only one exception, and that is in the genus Casto?·. In the genus Ondatra, the de ccnding mmus is but slightly twisted outwards, but in all the other Arvicolidw, whoso crania I have examined, it is rcnmrkably so, and in the genera Spalam and Ceomys, where this character is carried to the extreme, the descending ramus proj ects from the alveolus of tho long inferior incisors, in tl1o form of a rounded and almost horizontal plate. tIn aged individuals of some of tho species of Arvicolidw, the molar teeth possess short roots. In a ~~~111 of Ondatra How before m() I find all tho molars divided at tho base iuto two portions, which in all prolmblhty would have formed solid roots lmd tho animalli1'ed longer. MAMMALIA. . present tl1e characters above . 77 d h pomted out a d' · regar s t e cranium and lower . . . s Istmguishing the Arvi l"d proach is evinced Jaw, It IS only in the gcn N. co l m, and as Of I . . us eotoma that any ap- . . . t le vanous groups of the . . - Sclurtdce, so far as I Oldci Rodentz'a found i S do not occur in tb am aware, are chiefly confined to them ~ onth America, tile . e most southern. the M . me northern parts and ~a~~~n~ld T~ ~l~ecies of the famil~ MuriJ:·~t:~:~g~:·~~~~· and A.t·vicolid~ are tabl' h d .oz . Of the Leporidm I am . I etent sectJOns to those IS. e species-the Lepus Bm.ziliensi _acqumnted only with one well es- Amenca Australi " as p· 1 s, wluch however is not f' .1 • ' Isc Ier says th . b . ounu " m tota sout~ern parts, where the Cavies ' e~c _emg no Hare yet found in th remaming South Am . R and C!unclttllas appear to take th . 1 e more E I.. encan odents-certa' . etr pace. The c nmys, Das!Jp?·octa, Cceloa·em s 'In species of I-I!Jslricidce the and CMncltz'llidm, all posse o y and]' JJ'l!Jopolamus, together with the' 0 t dgcn~ra, or less . ~ s a pecu Iar form of skull . c o ontzdce approaclun5a- to .figs 1 J:>l t and of the lower J. a'v ( des 'b .1 • • , a e 33 and fi , more en eu m the "Mag·azine of N t I H'. < o-s. 23, Plate 34.) which I h ra . 1 . c . a ura 1sto " c F ave re y IOUnd In the North A .· ry, IOr •cbruary 1839 and I. 1 . abo meitcan or Old w ld ' w He 1 1s ~·ffi ve groups, I omitted the Caviilm b . o~· Rodents. In enumerating the I er somewhat from the rest-the 'o ccaus~ m the form of the lower jaw the toththemselves; but in the Chinchi~a~ ""'s:~ss,t In f~~t, a form oflower-jaw peculi:r o erT ahr e found . e ransttJons between one J'o d h !4 rm an t e e South American M ··d . collection, were none of tl utz m, whteh form the chief part of Mr D .. ' 1 1em procured f, h · anvm s t le exception of those from the G I . ~ut er . north than latitude 30o, with tl1e following localities. a apagos Archipelago. The species occur at WEST CO AST OF SOUTH AMERICA. GALAPAGos AncmPELAoo. Mus Jacobim. -- Galapagoensis. COQUIJIIDO. Mus Iongipilis. --Renggeri. EAST COAST OF SOUTH AMERIC.A. MALDONAllO, Mus decumanus. --muurus. --.Afusculus. -tumidus. --na utus. --ob curus. --arenicola. -- bimaculatns. --Darwinii. --flavcsccns. Rcithrodon typicus . .,. Seo Proceedings of the Zoological Society for April 9th, 1839, P· 61. |