136 WEALDEN AND PURBECK FOSSIL FISHES. 



large fish (PI. XXV, fig. 4), all the vertebral centra are of this type, except that 

 they are generally longer in proportion to their depth. There seem to be 34 or 

 35 in the abdominal region, but the number of caudals is uncertain. The centra 

 in the caudal region are scarcely longer than those in the abdominal region, but 

 they differ in exhibiting a slight lateral indentation above and below a rounded 

 median lateral ridge. The neural arches in the abdominal region are long and 

 slender, but most of them are broken and displaced, and obscured by the 

 overlapping intermuscular bones. The stout, gently-arched ribs are well shown, 

 reaching the ventral border, each impressed by a longitudinal groove. The neural 

 and haemal arches in the caudal region are firmly fixed in their sockets in the 

 centra. 



The remains of the clavicle (PL XXV, fig. 4, cl.) and supraclavicle (scl.) include 

 a wide, nearly smooth, exposed portion; and a large smooth plate of bone (x.) 

 shown behind the skull is probably the post-temporal, much expanded, as in 

 Thrissops. The long pectoral fin-rays (pet.) are very closely articulated distally. 

 The pelvic fins (plv.), with slender supports, are much smaller than the pectorals, 

 and are evidently inserted nearer to the anal than to the latter. Of the anal fin (a.), 

 only a fragment remains with at least 1 7 rays. As the anterior supports are 

 relatively long and stout, this fin must have been acuminate. The dorsal and 

 caudal fins are unknown. 



Scales are seen in some of Mr. Hooley's specimens, all large, thin, and cycloidal, 

 and deeply overlapping. They are most conspicuously marked by the concentric 

 lines of growth ; but some exhibit sparse pustulations, usually in rows, on the 

 smooth exposed face, besides fine fimbriations at the hinder margin (PI. XXIV, 

 fig. 7). _ 



Horizon and Localities. — Weald Clay: Atherfield, Isle of Wight; Berwick, 

 Sussex. 



Genus THRISSOPS, Agassiz. 



Thrissops, L. Agassiz, Poiss. Foss., vol. ii, pt. i, 1833, p. 12. 



(-leneric Glut racier*. — Head as in Leptolepis, but smaller, and the dentary bone 

 with a broader, less thickened elevation. Vertebral centra much strengthened by 

 secondary ossification in longitudinal ridges, of which the median lateral is usually 

 very prominent ; no centra much deeper than long. Ribs especially stout. Pelvic 

 fins relatively small ; dorsal and anal fins acuminate and opposite, the former small 

 and short-based, the latter much extended ; caudal fin forked. Scales thin, com- 

 pletely covering the trunk ; no enlarged or thickened ridge-scales. 



Type Species. — Thrissops formosus (L. Agassiz, Poiss. Foss., vol. ii, pt. i, 1833, 



