XV HVSTRICIDAE OR GROUND PORCUPINES 499 



sixth being very small. The clavicles are well developed. A 

 curious fact about C. villosus is that the acetabular cavity is 

 perforate (on both sides), or at least only closed by membrane. 

 In many forms of Eodents the bone is very thin in this region. 

 This fact perhaps lessens the significance of the perforation of the 

 acetabulum of Echidna (see p. 109). 



Of the allied genus Chaetomys, also Neotropical, there is but a 

 single species, which inhabits Brazil. It has a nearly completely 

 closed orbit, a feature which differentiates it from the last animal, 

 and one which also shows it to be a more modified form. Tlie 

 spiny covering is less pronounced than in its allies. 



Fam. 8. Hystricidae. — This family is characterised by the fact 

 that all its members possess spines ; but the tail, if at all long, 

 is not prehensile, and the soles of the feet are smooth and not 

 covered with rough tubercles, as in the Tree Porcupines of the 

 next family, Erethizontidae. The clavicle is less developed than 

 in the arboreal forms. In the organs of digestion there are 

 points of a family difference between the two groups of spiny 

 Eodents. The tongue has serrated scales arranged in transverse 

 rows, which are directed backwards. A gall-bladder, though not 

 always present, is sometimes found ; it apparently never exists 

 in the arboreal Porcupines and in Erethizon. The lungs show 

 a great tendency to subdivision, which appears to be especially 

 marked in the genus Atherunf. The caecum seems also to be 

 shorter in the Ground Porcupines. In Hystrix cristata the 

 small intestine measures 1 5 feet 7 inches ; the caecum, 8 inches ; 

 the large intestine, 4 feet 4 inches : — in Atherura a/Ticana the 

 caecum measures 7-^ inches ; the large intestine, 1 foot 1 

 inches. The corresponding measurements of Syjietheres villosus 

 were : small intestine, 7 feet 3 inches ; caecum, 1 foot 4 inches ; 

 large intestine, 2 feet 7 inches. In Erethizon the caecum is 

 2 feet 4 inches in length. These differences are too large 

 and too constant in a number of presumedly allied forms to be 

 overlooked. 



Mr. Parsons has directed attention ^ also to a number of 

 muscular differences, such indeed as might be expected to occur 

 between animals of such different habits. 



The genus Hystrix embraces the better-known Porcupines. It 

 is a genus of wide range, extending from the East Indies to Africa, 



^ Proc. Zool. Soc. 1894, pp. 251, 680. 



