6 FOSSIL FISHES OF THE ENGLISH CHALK. 



maximum deptli of the trunk, wliicli nearly equals tlie length from the pectoral 

 arch to the base of the caudal fin. External head-bones only partially rugose, the 

 hinder expanded portion of the frontals being quite smooth, and the front edge of 

 the supraoccipital crest not thickened or ornamented. Antorbital cheek-plate 

 slightly deeper than broad, its depth about equalling that of the orbit. Dorsal 

 fin with 6 short and stout spines, which are nearly smooth ; anal fin arising much 

 nearer to the pectorals than to the caudal. Scales very feebly rugose, or sometimes 

 quite smooth, with slightly l)ent, not regularly curved hinder margin ; nearly 

 30 scales in each transverse series on the abdominal region. 



Description of Specimens. — The type specimen in the Brighton Museum does 

 not exhi1)it many of the essential characters of the fish. It is, however, sufficiently 

 complete for generic and specific determination. The gently rounded frontal 

 region of the skull is shown, and the large supraoccipital crest is ol)servable in 

 transverse section. The operculum is much flaked, but seems to have been smooth. 

 The scales are well displayed, exhibiting their proportions and the typical rhombic 

 exposed area, with a very slight rugosity at its hinder border. The front part of 

 the anal fin is preserved, and its remains extend to the hinder fractured edge of 

 the fossil. 



The only other important known specimens of this species are contained in the 

 collection of the British Museum. All these are very imperfect ; l^ut two afford an 

 approximate idea of the general proportions of the fish, while the others exhibit 

 most of the principal characters of its skeleton. 



The general proportions are best shown in the small specimen from the Chalk 

 of Sussex represented of the natural size in PI. I, fig. 1. I'he frontal profile of the 

 head is steep, and the back gradually rises to the origin of the doi'sal fin, Avhere the 

 trunk is deepest. The ventral margin of the Ijody forms a more gentle curve than 

 the dorsal border. The jaws are slightly pressed forwards in fossilisation, l)ut, 

 allowing for tliis, the length of the head with opercular apparatus is seen to equal 

 its maximum depth at the back of the occipital crest. Its length also somcAvhat 

 exceeds two thirds that of the trunk from the pectoral arch to the l)ase of the 

 caudal fin. 



The roof of the skull is shai'ply bent aliove the middle of the lai'ge orbit, and its 

 short hinder portion is surmounted by a deep triangular supraoccipital crest 

 (PI. I, figs. 1, 3, socc), which is strengthened by one oblique ridge. The muscles 

 of the trunk must have extended forwards over this portion of the skull as far as a 

 sharp ridge which is inclined backwards on each side from the middle point at the 

 hinder l)order of the frontal region (fig. 3, _/>.). The frontal bones (/r.) are 

 smooth and a. little tumid, impressed with a few reticulating grooves, and by a pair 

 of lateral, longitudinal sensory canals. Tliere is also a large supraorbital flange 

 (sph.), apparently of the same element, Avhich bears a finely rugose ornamentation. 

 Anteriorly the frontals taper, and overlap the narrow mesethmoid, which is almost 



