OLIGOPLEURJDJL. 
491 
The undefined generic name of Holochondrus is mentioned by 
Thiolliere immediately after Oligopleurus (Poiss. Foss. Bugey, pt. i. 
1854, p. 4), but nothing is known of the fish to which it refers,, 
Genus OLIGOPLEURUS, Thiolliere. 
[Ann. Sci. Phys. & Nat. Lyon, [2] vol. iii. 1850, p. 154.] 
Head large and snout pointed ; maxilla arched, with convex oral 
border; dentary truncated in front and mandible prominent; all 
teeth minute. Gill-rakers large and closely arranged. Vertebral 
centra in part deeper than long, longitudinally striated, but without 
lateral pits; ribs delicate. Pin-rays robust, all closely articulated 
and divided at some distance from the base ; fulcra conspicuous on 
the median fins. Dorsal and anal fins short-based, nearly or com¬ 
pletely opposed ; caudal fin gently excavated at its hinder border. 
Scales large. 
The cranial osteology of this genus is imperfectly known and best 
illustrated by the specimens enumerated below from the Wealden 
of the Isle of Wight and the Purbeckian of Swanage 1 . The head is 
of triangular shape and much laterally compressed. In Oligopleurus 
vectensis the cranial roof is broadest at the occipital border and 
gently arched from side to side ; it exhibits a longitudinal median 
depression attaining its maximum depth and greatest breadth 
immediately behind the interorbital region, and gradually becoming 
shallower in front and behind. There is no ossified interorbital 
septum, and the middle portion of the parasphenoid is narrow, 
apparently without teeth. The mandibular suspensorium is some¬ 
what inclined forwards, but the gape of the mouth is wide, the 
mandible projecting in front of the premaxilla. The hvoman- 
dibular is much constricted mesially, laterally compressed above, 
and antero-posteriorly compressed below. The quadrate is fan¬ 
shaped, constricted immediately above the articular condyle, and 
with a cleft near its hinder border for the reception of the sym- 
plectic; there is also a curious inwardly-directed process from the 
base of the condyle. There are the usual three pterygoid elements 
on each side, but rather delicate. The premaxilla is very small. 
The maxilla is a long, narrow, laterally compressed, and gently 
arched bone, of almost uniform depth, except in its anterior third, 
which gradually contracts and ends in a stout inwardly-directed 
process ; the oral border is convex, and the hinder two-thirds of the 
1 See figures and description in Proc. Zool. Soc. 1890, pp. 346-350, 
pis. xxviii.. xxix. 
