xv HYSTRICIDAE 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 Rodents 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. The 
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 
Rodents. 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 Hrethizon. The lungs show 
a great tendency to subdivision, which appears to be especially 
marked in the genus Atheruwra. The caecum seems also to be 
shorter in the Ground Poreupines. In Hystria cristata the 
small intestine measures 15 feet 7 inches; the caecum, 8 inches ; 
the large intestine, 4 feet 4 inches:—in Atherura africana the 
caecum measures 74 inches; the large intestine, 1 foot 10 
inches. The corresponding measurements of Synetheres villosus 
were: small intestine, 7 feet 3 inches; caecum, 1 foot 4 inches; 
large intestine, 2 feet 7 inches. In Zrethizon 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, 
1 Proc. Zool. Soc. 1894, pp. 251, 680. 
