OCR Text |
Show ZOOLOGY Or TUE VOYAGE 0 F THE BEAGLE. r:.eneral Observations upon t!teforegoing Species of M~t1·idce. In the foregoing descriptions I have endeavoured to convey an idea of the characters of the species of mice submitted to me for examination and description, by Mr. Darwin: there are, however, some points upon which. I have be~n. silent iu my descriptions. I allude to the characters observable m the dentltwn. I have omitted to notice the various modifications in the structure of the molar teeth, because I found it would lengthen the descriptions to no good purpose, inasmuch a of almost all the species I have made outlines of the molars, which will convey a more clear idea than any verbal description can do. Upon an inspection of tho Plates, it will be seen, that by far the greater portion of the teeth figured, may be referred to one particular type of form or pattern, and tlrat this pattern does not agree with that observed in the molars of Mus Rattus, ]}f. decwnanus, or JJ{. musculus, whilst these three species agree essentially with each other. In the young Black Hat (JJ'lus Rattus), before the teeth are worn, the two anterior molar· teeth, on either side of tho upper jaw, present three longitudinal rows of tubercles, a central series of larger tubercles, and on each side of these, a row of smaller ones. The front molar has three of the larger tubercles arranged along the middle of the tooth; three smallet· ones on the outer side, and two, on the inner side. Tire secouu molars lrave two central tubercles, two outer, and two inner ones. 'l'h~ posterior molar is nearly round , the body of the tooth consists of three principal tubercles, and one small tubercle, situated on tho inner and anterior porlion of lhe tooth. The corresponding teeth in the young of Mns bimaculatus present a very different appearance; the molars, instead of having three longitudinal rows of tubercles, have only two. An idea of tlte appearance of these teeth may be formed by removing th~ inner row of tubercles from the molars of Mus mltus. We should then have, as in Mus I.Jimacttlatus, molars of a narrower form, the first tooth presenting six tubercles, the second, four; and the posterior tooth devoid of the small inner lobe; the opposing tubercles of each tooth, however, in M. bi~niculal1ts, are of equal size. The molars of the lower jaw of Mus birnaculatus agree with those of M. Rattus as to the number of tubercles which they possess; they are, however, proporlionately longer and narrower, and, when a little worn, these teeth, as well as those of the upper jaw, difi'er considerably from those of J.lf. Ratlus. In the last named animal, when the molars are slightly wom, the ridges of enamel run completely across the tooth, as in Figs. H3 and 19, Plate 34. Such is not the case MAMMAUA. in M. bimaculatus at any age As · soon as the molar te tl enamel penetrate the body of the t tl . e 1 are worn, the folds of 7!) . 1 oo 1 on each side and th f nate Wit 1 those of the other · ., I. ' ose o one side alter-zT ,-m met, t wy very ne ·I .uamsters (C1·icetus). ar Y resemble those of the 1 have selected the molar teeth of 718" R . pan·s on, sm· ce I happened to pos .Lu.u.s altu·s. and lJII. bl ·m acu la tus fio r com-a d u I t states of each But had I sess specimens dtspl · b 1 1 1 ayrng ot 1 the youn~Y and . · se ectet , on the on h 1 1 ° specres brought fi·om South A .· . b M . e anc, a most any of the h meuca y r Darwr I t e lJfus m~tsculus o1• M de I · n, anc • on the other hand ] . . . . cmnanus, should have h d t . ' ( tstmctrons-the former agreein . d . . . a o powt out the same with M. RattztS. g In entitron ·wtth M. bimaculatus, and the latter The differences pointed out between tl I those of M. bimacutatus I can 't b . re mo ~r teeth of JJ'lus Rattus and Wol'ld species of Mus ~hid I ~o, ut consrdcr as Important, since all the Old ] J lclVe yet had an opi)Ol'tun 't f .. t Iey are numerous) aO'r·ee esscnf ]] . h . l Y o exammmg (and S. America (exceptinao M )l,r Jla y wdr\, the former, whilst the only ~Ius from o · .Lr.Luscu us an HI decmn h · h . to all parts of the world) in which I h . anus, w tc are earned in ships M. Rattus is the 718" u ave as yet found molar teeth like those of ' .Lr.LUS .Lr.taurus and th' 't h b of lJf. rlecumanus. ' IS I as een stated is possibly a variety Although as yet I lrave not met with s .· . the characters of tho South A . JIIK ~ecies m the Old World possessing several have come under mb encan. '.L'llndce ' among tho' f N h A . ~eo ort merrca, and the species of N.eoto my ot ~elrvabtwn. The Mus leucopztS, Symidon ltispidurn, · ma cer am y elong t tl species of the Galapaoo I 1 1 d . .0 le. same group,* as does also the pagoeusl.s . ,..., s anc s, escnbed m thrs work under the name G al a- These considerations h · d 1 from those of th Old 'V ·Idave m ucet me to separate the South American mice e Ol ,-01' rather from that group of which M. decumanus may .be regarded as the ty}le,-and to place them, t I . A mencan species as agree w1'th th · oget 1er wtth such North name Hesperomys.-j" em 111 dentition, in a new ge nus 1u. earm• g t 11 e Whether this grou b fi d 1 venture to sa . . ? e con ne to t le Western hemisphere or not, I willuot their h. f ) , bnt ~ thmk I mny safely affirm that that portion of the globe is c Ie metropolis. The species of the g R . dentition is 1 . enus esperornys, whiCh depart most fi·om the type-wlwse east hke figs. 5, a, and 5, b, Plate 33. or 6, a, and 6, b, of the ·• I am ac!Juaintcd with seven No tl A . S . peromys. r 1 morJCan pccws of Murida>, all of which possess tllC dentition of Hc.·- t 'Eu'~~"Epos, West, and Mvs. |