364 



2007. The skull of a Guinea Pig (Cavia aperea), vertically and longitudinally 

 bisected. 



The alisphenoid is perforated by an enormous foramen ovale, a thin tract of bone circum- 

 scribing it behind and separating it from the petrosal bulla. 



Purchased. 



i 



Genus Chinchilla. 

 Dental formula : t ^, p \^, m ^5=20. 



2008. The skeleton of the Chinchilla (Chinchilla lanigerd). 



The vertebral formula is : 7 cervical, 13 dorsal, 6 lumbar, 3 sacral, and 23 caudal, which 

 is probably one short of the true number. The common ridge of the anapophysis and met- 

 apophysis rises above the diapophysis of the ninth dorsal, bifurcates on the tenth, and sepa- 

 rates into the two accessory processes on the eleventh vertebra : they are continued distinct 

 to the penultimate lumbar. Seven pairs of ribs directly join the sternum, which consists of 

 six bones. The clavicles are here complete : the acromion is long, slender and bifurcate, the 

 lower and longer branch descending and curving forwards. The humerus is perforated between 

 the condyles, but not above the inner condyle. The radius and ulna are in contact through- 

 out the greater part of their length, but are not anchylosed. The scaphoides and lunare are 

 connate. The pollex is retained with both phalanges, but does not reach to the end of the 

 metacarpal of the index. The hallux is wanting in the hind foot, but there is a long ento- 

 cuneiform bone and accessory ossicle on the inner side of the joint, between the astragalus 

 and scaphoides. The fibula is very long and slender, but is articulated to the tibia only at 

 its extremities. There is a fabella behind each condyle of the femur. The auditory bulla is 

 enormous ; it is formed by the expansion of the mastoids, tympanics and petrosals, the mas- 

 toids being pushed to the upper surface of the cranium, where they articulate with the 

 parietals, interparietals and superoccipital, and are crossed at the lower border by the clamp- 

 ing processes of the squamosals, which pass backwards between the mastoid and tympanic, 

 and bend down abruptly behind the auditory meatus, and between it and the posterior ex- 

 panded part of the petrosal. There is a large oblong vacuity between the squamosal and 

 tympanic, where the cerebral cavity is closed only by membrane. The area of the auditory 

 meatus is a narrow oval, being, as it were, pressed in laterally : the direction of the amal is 

 upwards and a little backwards. The sagittal suture, as well as the frontal one, remains. 

 The malar bone extends more forwards, and forms a larger proportion of the zygoma than in 

 the Guinea Pig. The antorbital vacuity is equally extensive, but it is deeper than it is wide. 

 The fossa and communication with the nasal cavity above the molar protuberance is narrower 

 than in the Guinea Pig. The nasal processes of the premaxillaries extend beyond the nasals. 

 The outer side of the lower jaw is convex and regularly swollen by the protuberance answer- 

 ing to the bases of the curved molares, but has not the distinct ridge which characterizes the 

 lower jaw in the Cavies. The angle is considerably produced backwards, but is much more 



