PROFESSOR OWEN ON THE AYE-AYE. 79 



^13. Comparison of the Skeleton. 



The number of the ' true 'vertebrse, and of each of their three kinds (c 7, ci 13, Z 6=26) 

 in Chiromys, agrees with that in Lemur, Tarsius, and Sciurus (as exempHfied by Sc. 

 bicolor) : this, indeed, is a common formula in ordinary quadrupeds. But both in the 

 Lemurida and Sciurida there are great differences: in Stenops tardigradus, e.g., the 

 formula is c7, d \6, Z 8=31 ; and in a large flying Rodent {Anomalurus pelii) it is 

 c7, cZ15, n0=32. 



The affinities of Chiromys are shown rather by the structure of the vertebrae. In the 

 agile Squirrels, which, on the ground, progress by bounds with considerable flexure of 

 the spine, such action is indicated by the much stronger inclination of the vertebral 

 spines before and behind the eleventh doi'sal, towards that centre of the spinal inflexions, 

 than is seen in Chiromys. The diapophysis, moreover, is distinctly developed on the 

 vertebra, e. g. the ninth dorsal in Sciurus bicolor, the tenth in Anomalurus, where the 

 combined met- and an-apophyses form the ridge above it ; and the diapophysis 

 continues to be distinctly developed in the succeeding vertebrae, in which both the 

 metapophysis and anapophysis have become distinct, as is the case in most, if not all 

 Rodents : whereas in Chiromys, as in other Quadrumana, the diapophysis is suppressed, 

 or nearly so, in the last two dorsal vertebree in a degree which is misleading, and has 

 misled, in the attempt to determine the homologies of the lumbar transverse processes. 

 These processes in the Sciuridce are longer and more inclined forward than in Chiromys 

 and the Lemuridm. The pleurapophysial parts of the cervical transverse processes are 

 more developed in the 3-6 cervical vertebrae in Sciuridce — the axis is longer in proportion 

 to the atlas — than in Chiromys. 



The chevron-bones (haemapophyses) are two in number in the caudal region, and are 

 confined to the interspaces between the third and fourth, and fourth and fifth vertebrae 

 in Chiromys ; they are similarly restricted in number and position in the long-tailed 

 LemuridcB : in the Squirrels and other long-tailed Rodents the haemal arches may be 

 traced along a much longer proportion of the caudal region. 



In comparing the skull of the Aye-aye with that of a Squirrel or other Rodent of equal 

 bulk of body, the first great distinction is shown by the superior size, both absolute and 

 relative, of the brain-case ; especially in the part due to the size and convexity of the 

 parietal and frontal bones ; making the region, which is low and flat in the Sciuridce, 

 an expanded convex dome in the Aye-aye. The foramen magnum is relatively 

 larger to the cranial cavity in Rodents than in the Aye-aye ; it is also vertical in position, 

 looking directly backward ; and the superoccipital does not bulge out beyond and behind 

 it, as in Chiromys, in which the plane of the foramen is turned as much downward as 

 in other Lemurida. The cranial vertebrae follow each other in a more straight line, in 

 Rodents ; which, with the position of the occipital condyles and aspect of the foramen 

 magnum, causes the premaxillary end of the skull to be on nearly the same line as the 



