PROFESSOR PETERS ON THE GENUS PECTINATOR. 405 
Dr. v. Heuglin observes that he very often saw Pectinators lying on the rocks, and 
with their bellies so much flattened that they were hardly to be distinguished from 
the rocks. He supposes, therefore, that their abdominal muscular system is endowed 
with a peculiar extensile power; however, it does not show any striking peculiarities. 
Brain and Organs of Sense. 
The brain (Pl. L. figs. 7-10) of Pectinator is very differently shaped from that of 
Dipus, flatter and longer, more like that of Habrocoma, to judge from the cranial 
cavity of this latter genus. The cerebrum is lozenge-shaped, without any vestige of 
circumyolutions, with the anterior longer borders concave ; the lobus hippocampi below is 
hardly distinguished by a flat horizontal furrow. The corpora quadrigemina are exposed 
between the hemispheres and the cerebellum. The hypophysis is nearly quadrangular, 
with pointed angles. I have no opportunity of comparing it with the brains of other 
genera of the family of Hystricide of Waterhouse. But it will be seen that there is a 
very great difference between the brain of Pectinator (Habrocoma, Chinchilla) and that 
of the Porcupine’. The difference is indeed so great, that, in a classification based on 
the formation of the brain, it would be hardly possible to leave them together in the 
same family. 
The eyelids are hardly separated at the inner commissure by a small notch, and are 
provided with very short black lashes. The caruncula lachrymalis and the plica semi- 
lunaris are little developed. The eyeballs are of moderate size, their transverse diameter 
not exceeding 8 millims. The lens is nearly spherical, anteriorly much less convex than 
posteriorly ; its axis is 5 millims. long, its transverse diameter 8 millims. 
The annulus tympanicus (PI. L. fig. 11), which in the Dipoda is very little elevated, 
is, as in the Hystricidz, very prominent, and with its lower margin united to the bulla 
by radiating long lamella. As in nearly all the Hystricide’, the malleus and incus are 
coalescent, and the head of the malleus is large, elongate, club-shaped (PI. L. fig. 12), 
and, on the middle of its inferior side, concave. In this respect also the Ctenodactyli 
show a very important divergence from the family Dipoda. The cochlea is elongate, 
as in Chinchilla, Echinomys, Petromys, Habrocoma, Cavia, &c., and forms a little more 
than four windings, the first of them being very much longer than the rest. Likewise 
the proportions of the semicircular canals are more similar to those of the Hystricide 
than of any other family of Rodents. 
The nasal cavity is very like that of Chinchilla and Habrocoma, in consequence 
narrower, and the conche less complicated than in the Dipodina. 
* Of. Owen, Anatomy of Vertebrates, iii. p. 110, fig. 77. 
* Cf. Hyrtl, Vergl. anatom. Untersuch. iiber das Gehororgan des Menschen und der Saugethiere. Prag, 1845. 
Taf. iv. figs. 29-34. 
VOL. VII.—PaRT Vv. January, 1871. 3K 
