492 
ACT1X0PTERYGII. 
superior border exhibit a deep narrow depression, overlapped by 
two supramaxillary bones, which are arranged like those of Pholi- 
dophorus and the Clupeoids. Only two elements have been observed 
in the mandible, a short articular bone behind and a long dentary 
forwards; these meeting in a high coronoid elevation, which is 
chiefly formed by the dentary. The teeth are small and conical, 
and seem to have been more or less clustered on the margin of the 
jaws. The cheek-plates are very delicate, but appear to have 
covered the whole of the circumorbital space. The opercular 
apparatus is complete, and the preoperculum is considerably exposed, 
marked with radiating ridges. Branchiostegal rays are also well- 
developed, but no gular plate has been observed. The branchial 
arches are remarkable on account of the great size of the gill-rakers, 
which are smooth pointed stylets of bone, slightly constricted near 
the base and arranged in very close series. 
The vertebral centra are completely ossified, amphicoelous, usually 
deeper than broad, and marked on the side by fine transverse 
striations extending between a thickened rim anteriorly and pos¬ 
teriorly ; a pair of deep pits on the upper aspect accommodates the 
neural arch, and there is a similar pair of pits on the ventral 
aspect for the insertion of a haemal arch. The only traces of 
attached peripheral elements hitherto observed consist in a small, 
faint, rounded pit or rugosity on some of the anterior centra, which 
may have supported an intermuscular bone. The first vertebral 
centrum, articulating with the basioccipital, is composed of two 
thin discs fused together, the anterior supporting the neural, the 
posterior the haemal arch. The other centra exhibit no suture. 
The neural spines are free from their supporting arches in the 
abdominal region and comparatively robust; those of the caudal 
region are slender and fused with the delicate low arches, which 
have prominent zygapophyses. The ribs are short and delicate. 
Oligopleurus esocinus, Thiolliere. 
1850. Oligopleunis esocinus, V. Thiolliere, Ann. Sci. Phys. & Nat. 
Lyon, [2] vol. iii. p. 154. 
1854-73. Oligopleurus esocinus , V. Thiolliere, Poiss. Foss. Bugey, 
pt. i. pi. ix., pt. ii. p. 21. 
Type. Nearly complete fish ; Lyons Museum. 
The type species, attaining a length of at least 0*45. Length 
of head with opercular apparatus nearly equal to the maximum 
depth of the trunk and almost one-quarter of the total length of the 
fish : depth of trunk at pectoral arch comprised nearly six times in 
the total length. Anterior abdominal vertebral centra somewhat 
