PROFESSOR OWEN ON THE AYE-AYE. 79 
§ 13. Comparison of the Skeleton. 
The number of the ‘ true ’ vertebra, and of each of their three kinds (c 7, d 13, 16=26) 
in Chiromys, agrees with that in Lemur, Tarsius, and Sciurus (as exemplified by Se. 
bicolor) : this, indeed, is a common formula in ordinary quadrupeds. But both in the 
Lemurideé and Sciuride there are great differences: in Stenops tardigradus, e. g., the 
formula is c7, d16,18=831; and in a large flying Rodent (Anomalurus pelii) it is 
e7, 415, 110=32. 
The affinities of Chiromys are shown rather by the structure of the vertebre. 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 dorsal, 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 vertebre, 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 vertebra 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 Sciwride are longer and more inclined forward than in Chiromys 
and the Lemuride. The pleurapophysial parts of the cervical transverse processes are 
more developed in the 3-6 cervical vertebre in Sciuride—the axis is longer in proportion 
to the atlas—than in Chiromys. 
The chevron-bones (heemapophyses) 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 
Lemuride: in the Squirrels and other long-tailed Rodents the hemal 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 Sciuride, 
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 Lemuride. The cranial vertebre 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 
