PYCNODONTIDJE. 
195 
to the presence of a bony base in the one case and its absence in 
the other. Moreover, in comparing the detached vomers and 
splenials of Pycnodonts, the smaller specimens cannot be compared 
as a whole with the larger ones—they must be compared only with 
that portion of the latter which happens to be of corresponding 
size. 
The notochord is always persistent, and no ossifications which 
can be ascribed to its sheath have hitherto been observed. The 
neural and haemal arches, however, are invariably well-ossified, and 
their bases are so much expanded that they sometimes ( Pcdceo - 
balistum , Pymodus ) completely encircle the notochord. Ribs are 
present; and those of Gyrodus and Palceobalistum are distinctly 
observed to exhibit the laminar expansion so characteristic of the 
neural spines throughout the trunk and of the haemal spines in the 
caudal region. This expansion is commonly confined to the anterior 
edge of each spine, but in Gyrodus and Palceobalistum it occurs 
both anteriorly and posteriorly ; and in the last-mentioned genus 
the laminae of adjacent spines are actually united for a considerable 
length in a jagged suture. The neural spines in the abdominal 
region extend to the dorsal margin of the fish, and are often 
thickened apparently for the support of the ridge-scales. The 
haemal spines supporting the caudal fin are also more or less 
expanded, but never fused together. No intermuscular bones have 
been seen. 
The precise mode of suspension of the pectoral arch to the 
cranium still remains to be discovered; but the clavicle is well 
known, and some specimens of Gyrodus appear to exhibit a supra- 
clavicle extending towards the back of the cranial roof. The 
clavicle is much vertically elongated, tapering above, and with a 
spatulate expansion below; a very small surface is exposed on the 
flank of the fish. There is no infra-clavicle. The scapula and 
coracoid cannot yet be described, but the pectoral fin is attached 
just above the inferior expansion of the clavicle. The base of this 
fin forms a little lobe, distinctly covered with thin rounded scales 
in Gyrodus; and a single series of seven or eight basals can be 
readily counted in specimens both of Gyrodus and Palceobalistum , 
while some vacant space may have been occupied by still more of 
these elements. The pelvic fins, though often quite rudimentary, 
are exhibited in all the genera of which the trunk is satisfactorily 
known. The rays of the dorsal and anal fins are equal in number 
to their supports, which are more numerous than the vertebral 
arches in the same region; but each fin-ray is sometimes liable to 
be subdivided into its right and left halves by crushing, and there 
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