26 FOSSIL FISHES OF THE ENGLISH CHALK. 



base of the caudal fin. The depth of the caudal pedicle seems to be contained 

 nearly three times in the maxinuim depth of the abdominal region. The fish must 

 have been much laterally compressed. 



As shown by the type specimen and by B. M. no. P. 1952 a, the head in side- 

 view is triangular, with a sharply pointed snout. The cranium is bent above the 

 middle of the orbit ; and its short hinder portion, over which the muscles originally 

 extended, is surmounted by a deep and triangular, laminar, median longitudinal 

 crest (PI. VIII, fig. 6, soec). The frontal profile is steep and straight. The 

 frontal bones are partly shown from above in PI. VIII, fig. 7 (/r.), and do not bear 

 any large slime-cavities. The orbit is not very large, its depth being less than half 

 that of the head without the crest. The cheek-plates are imperfectly known ; but 

 there is a large anterior suborbital (PI. VIII, figs. 6, 7, ao.), which is ridged on its 

 outer face l)y the radiating branches of the sensory canal and is strongly serrated 

 on its loAver margin. The mandibular suspensorium is inclined forwards so that 

 the articulation of the mandible is not behind the middle of the orbit. The 

 long, slender premaxilla (PI. VIII, fig. 6, pm,v.) completely excludes the maxilla 

 (mx.) from the gape, as usual ; and the latter element terminates behind in a 

 smooth triangular expansion, Avhich is smaller in a typical specimen (B. M. no. 

 33230) than in the doubtful small fish from Burham (PI. VIII, fig. 6, m,v.). The 

 large hinder supramaxilla is ornamented with longitudinal rugge on the left side 

 of B. M. no. 33230, while in the Burham specimen it is smooth {sm,v.). When 

 well preserved, as in no. 33230 (PI. VII, fig. 7, md.), the mandible exhibits a 

 deep, longitudinally extended fossa on its outer face for the accumulation of 

 slime round the sensory canal. The bounding ridges are coarsely crenulated. 

 The teeth are minute, uniform in size, and clustered in both jaws. 



The opercular apparatus is best known by fragments on the right side of B. M. 

 no. 33230. The preoperculum is deep and narrow, curved rather than sharply 

 bent at its angle, which does not bear a spine. It is traversed by a deep groove 

 for the sensory canal, and the bounding ridges are coarsely though regularly 

 serrated. The operculum, also partly shown on the left side of the same specimen 

 (PL VII, fig. 7, op.), must have been shaped nearly like that of Hoploj^teri/x, but 

 with only one long and acute spine projecting from its hinder margin in the upper 

 portion. Its outer face is covered with delicate rugse which tend to radiate back- 

 wards and downwards ; and with this ornament are interspersed in the hinder 

 half a few short radiating ridges, of which the largest passes into the prominent 

 spine. The suboperculum is not satisfactorily known, but the interoperculum is 

 ornamented with coarse radiating ridges. Five branchiostegal rays are observable, 

 but there were probably more. 



The vertebral column seems to comprise 10 vertebra; in the abdominal region, 

 and probably about 14 in the caudal region. So far as known, the vertebrae 

 resemble those of Bcri/x and Iloplopteriix. A few of the caudals are especially 



