32 FOSSIL FISHES OF THE ENGLISH CHALK. 



(fig. 1, pm.v.) seem to be fused into a continuous mass with the mesethmoid and 

 vomer. The bone thus formed is expanded and obtusely rounded in front, while 

 its oral face is covered with a dense cluster of small, l^luntly conical or hemi- 

 spherical teeth. The mandible (d.) is deepest in the coronoid region, and tapers 

 towards the symphysis with a characteristic curvature. The oral face of the 

 dentary bone is somewhat expanded, and covered with a cluster of obtuse teeth 

 resembling those of the rostrum but smaller. 



The j^reoperculum (fig. 1, jJ0/>.) is well preserved in the type specimen, attached 

 to the hinder border of the mandibular suspensorium. It is a rather stout bone, 

 with a thickened straight anterior margin, and a small, semicircular posterior 

 expansion. The operculum (fig. 1, op.) is a very small bone, constricted just below 

 its thickened suspensory articulation and slightly expanded distally. 



There are traces of verteln'^ in the type specimen, but these are too imperfect 

 for description. 



Horizon and Localities. — Type specimen from zone of IiJn/ii(diouella Cuvieri or 

 Terehratulina gracilis : Houghton, Sussex. Another specimen probably from same 

 horizon near Dover. 



Suborder ISOSPONDYLI. 



Family ScorELiD.E. 



Genus SARDINIOIDES, ^\. von der Marck. 



Sardinioides, W. von der Marck, Zeitschr. deutsch. geol. G-es., toI. x, 1858, p. 245. 

 Dermatoptychus, W. von der Marck, Palaeontogr., vol. xv, 1868, p. 287. 



Generic Characters. — Head large, and maximum depth of trunk at origin of 

 dorsal fin. Cranial roof flattened, parietal bones meeting in middle line, and inter- 

 orbital region wide ; mandibular suspensorium nearly vertical ; jaws moderately 

 stout, and maxilla expanded behind; teeth minute and clustered; cheek scaly. 

 Branchiostegal rays about 9 in number. Vertebrte between 30 and 40 in number, 

 about half being caudal ; the centra longitudinally striated and not longer 

 than deep; ribs deHcate. No fin-rays elongated, but two or three in front of 

 median fins undivided and spinous. Pectoral fins delicate and small, inserted 

 well above ventral l)or(l(M'; })elvic fins larger and stouter, with about 7 rays, 

 opposed to the dorsal fin, which is nearly median and comi)rises from 10 to 

 18 rays; anal fin liehind dorsal, at least as deep as long, with about 10 rays; 

 caudal fin slightly forked. Scales large and uniform, finely serrated on the liinder 

 border, not deepened or thickened along the course of the lateral line. 



