PROFESSOR PETERS ON THE GENUS PECTINATOR. 401 
much less distant from each other, and less convergent in front than in that genus—the 
interspace between the last molars of Ctenodactylus being twice as wide as, in Pectinator 
only a little wider than, one of these teeth. The first upper molar (which is wanting in 
Ctenodactylus) is the smallest ; it is not half so large as the second one, and has its crown 
of an irregular quadrate form with pointed angles; the second is hardly smaller than 
the third, and has a deeply indenting fold of enamel only on the outer side, being thus 
similar to all the upper molar teeth of Ctenodactylus. 'The third and (the largest) fourth 
molars have a deep indenting fold on either side; and the fourth has, besides, on the 
outer side a small posterior second indentation. The first lower molar is nearly tri- 
angular, and much smaller than the corresponding upper one; the second has, like 
all the lower molars of Ctenodactylus, only one indenting fold of enamel on either 
side; but the third and fourth molars have two folds on the inner side, and one on the 
outer side. 
In both genera the molars are imperfectly rooted, which constitutes a difference not 
to be overlooked between them and the Octodontes and Chinchille’. 
Skeleton. 
I have first to point out the cranial differences of Pectinator from Ctenodactylus, 
which, although very important, are hardly more striking than those to be found 
between the different genera of Chinchillina. 
The whole skull of Pectinator is more flattened and has the upper profile less arched ; 
the free orbital process of the malar beneath the lachrymal is more pointed; the 
temporal fosse are narrower and less deep; the interparietal is broader behind than in 
front, and also absolutely broader than in Ctenodactylus, separating the parietal from the 
supraoccipital; the zygomatic arches are less expanded; the fossa lachrymalis of the 
lachrymal is less deep; the semicanal of the maxillary is prolonged backwards by a 
fronto-maxillary fissure, as in Lagostomus, almost as far as the ala parva of the sphenoid ; 
the palate between the molar teeth is much narrower, hardly wider behind than in 
front; and the postpalatal emargination is much deeper, extending as far as the middle 
of the last molars. The tympanic bull are also comparatively larger than in Ctenodac- 
tylus; but the meatus auditorius externus has the same direction, and is in the same 
manner elongated by an inferior semiannular osseous appendage, as in Cfenodactylus. 
Compared with other Rodents, the skull of Pectinator shows still more than that of 
Ctenodactylus the distance of this group from the Dipodes, and their close relationship 
to the Hystricide of Waterhouse. ‘The shortness of the lower part of the intermaxil- 
laries, and the short-rooted incisors, render the more lengthened snout much more 
similar to that of Habrocoma, Chinchilla, and Lagotis than to that of Dipus, Scirtetes, 
and Pedetes. The triangular infraorbital openings, in form and size, resemble rather 
1 Tt has been asserted that the Dipodes are provided with rooted molar teeth; and I find this so in the 
restricted group of Dipus, but not in Pedetes, which has rootless molars. 
