DERCETID^. 
185 
quarter of the cranial length ; the length of the head with opercular 
apparatus contained between four and five times in the total length 
of the fish to the base of the caudal fin. Anterior abdominal 
vertebral centra about three times as long as deep, those beyond 
less elongated, and those in the hinder half of the abdominal region 
with large transverse processes. Pelvic fins in advance of the 
middle of the trunk; [other fins unknown]. Dermal scutes of 
moderate size and narrow, more or less ornamented with short 
ridges and spinous tubercles, those on the dorsal and ventral 
borders of the caudal region also bearing a slender backwardly- 
directed uncinate spine. 
To this species Agassiz originally referred the specimens from the 
Chalk of Lewes described and figured as Murcena ? lewesiensu by 
G. A. Mantell, Loss. South Downs (1822), p. 232, pi. xxxiv. fig. 11, 
pi. xl. fig. 2. These fossils subsequently proved to be merely worm- 
burrows lined with various scales of fishes (W. Davies, Geol. Hag. 
[2] vol. vi. 1879, p. 145); and similar specimens from the Turonian 
of Bohemia were described under the name of Lepidenteron longis- 
simum (A. Pritsch, Eept. u. Pische bohm. Xreideform. 1878, p. 19, 
fig. 45). 
Form. Log. Senonian and Turonian : S.E. England. 
4132 - 33 . Type specimen, in counterpart, comprising the head and 
greater part of the trunk without fins, figured by Agassiz, 
tom. cit. pi. Ixvia. figs. ], 2, 6-8 ; Lewes, Sussex. 
ManteU Coll. 
4134 . Pragment of caudal region of large fish, figured by Agassiz, 
ibid. fig. 5 ; Lewes. Mantell Coll. 
49793 . Head and anterior part of trunk, in counterpart, the cranial 
roof shown of the natural size in PI. XII. figs. 4, 4 u ; 
Lewes. This cranial roof is fiaked in such a way that 
some pieces remain on one side of the fossil (fig. 4), and the 
rest on the counterpart (fig. 4 a). The postorbital region 
of the skull occujues one quarter of its total length, and is 
about as long as broad, wuth parallel sides and the 
occipital border excavated by a sharp re-entering angle. 
The frontal region between the orbits is excessively 
constricted in front. All the external bones are 
ornamented with a few delicate longitudinal ridges, 
occasionally subdivided into tubercles. When the roofing 
bones are removed, as in fig. 4, the well-ossified though 
small supraoccipital can be observed beneath, bearing a 
slight vertical median crest; but, as shown by the 
