CHARACTERS OP SOME HYSTRICOMORPH RODENTS. 425 



ancestral to the latter genus ; but with respect to the modifications 

 of the skull Gcdogenys is extraordinarily specialised and absolutely 

 isolated, nothing approaching an intermediate type between it 

 and Dasyin'octa being known. 



Family Chinchillid^. 



In all the classifications quoted above this family contains the 

 three genera Ohinchilla, Lagidiu'm, and Layostomus. Of these 

 Chinchilla and Lagostomus are the only ones I. have been able to 

 examine in a fresh state, Lagidium being known to me solely 

 from a dried skin, a skull, and the descriptions of other authors, 

 Avhich possibly from want of material do not contain the points I 

 want. But, judging from the ci-anium and the teeth and the 

 external characters revealed by the di'ied skin, Lagidium is more 

 nearly related to Chinchilla than it is to Lagostomus- — that is to 

 say, the family may be divided into two groups or subfamilies, 

 one containing Lagostomus alone, the other Chinchilla and 

 Lagidium. In using the external genitalia of the male as 

 perhaps the most important difterence between these subfamilies, 

 I assume provisionally that Lagidium will be found to agree with 

 Chinchilla. The characters may be tabulated as follows : — 



a. Penis normal in position and structure, glans stout and mcde- 

 rately long-, with well-developed glandular pouch. Fore feet 

 with trilobed plantar, bilobed carpal pads, and compressed 

 digital pads; hind foot with fifth digit retained, with a large 

 pad on the sole of the foot at its l)ase, digital pads com- 

 pressed ; principal digital brush on the second digit. Root 

 of ear raised externailj' so as to stand high above auditory 

 aperture. No coarse vibrissie on cheek. Molar teeth tri- 

 laminate. Postorbital area of skull sloping backwards so 

 that the weak occipital ridge is approximately on a level with 

 the middle of the orbit; naso-preraaxillary region not down- 

 curved ; " prepalatine foramina" long and " Steno's fora- 

 men" not remote from incisors Chinchillince. 



a'. Penis very abnormally placed, the prepuce being abdominal 

 in position ; glans exceedinglj' long and slender, with pouch 

 apparently undeveloped. 1^'ore feet with indistinctly lobed 

 plantar and carpal pads and digital pads not compressed ; 

 hind foot without fifth digit and no trace of isolated pad on 

 the sole. An immense digital brush on the third digit. 

 Root of ear not raised so as to conceal auditory orifice. A 

 mat of coarse vibrissse on the cheek ; normal genal vibrissa 

 stout. Molar teeth bilaminate. Postorbital area of skull 

 not sloping backwards, the very strong occipital crest as high 

 as the summit of the orbit; naso-premaxillary region arched 

 downwards; " prepalatine foramina" comparatively short 

 and " Steno's foramen " remote from the incisors Lagostomina 



Families Caviid^ and HYDROCHCERiDiE, nov. 



In the current text-books the Oaviidse contain the three genera 

 Cavia, Dolichotis, and Hgdrochcerus ; but the comparatively recent 

 severance of Cavia into some half-dozen genera — Cavia, Caviella, 

 Monticavia, Galea, and Kerodon — enhances the value of the 



