DERCETIS. 67 



The head (fig - . 2) exhibits the usual attenuated form, and its total length must 

 have equalled about three times its maximum depth at the occiput. The short 

 postorbital region of the cranium is indicated; and the left supraorbital flange of 

 the frontal (fr.) is seen from below, its edge bearing traces of the fine external 

 tubercular ornament. The very slender parasphenoid (pas.) is partly preserved in 

 its natural position. The remains of the hyomandibular, metapterygoid (mpt.), and 

 quadrate ('/"•) are thin, laminar bones; while the anterior edge of the preoperculum 

 seems to have been straight and vertical. The bone of the mandible is much fluked, 

 but on the right side its outer face is shown to be ornamented by very fine and 

 close tuberculated ridges, which are disposed longitudinally. The oral border of 

 the left dentary (d.) is impressed with a regular series of shallow sockets for small 

 teeth; and traces of the characteristic teeth themselves are scattered through the 

 chalk above. The fragments of bone just behind the head are not readily inter- 

 preted, but the imperfect left clavicle (el.) is distinguishable. 



The vertebral centra of the abdominal region are much-constricted cylinders, 

 their maximum depth exceeding half their length. They are all unsatisfactorily 

 preserved, and the nature of their arches is uncertain. The two examples marked 

 vb. in fig. 2, seem to exhibit the neural arch, which is produced postero-superiorly 

 into a short neural spine, antero-superiorly into a shorter zygapophysial articula- 

 tion. The imperfect centra further back, represented in fig. 2 a, bear along their 

 entire length a laminar process, which terminates at its free edge in sharp digita- 

 tions : whether this is the transverse process or a neural or haemal arch, cannot 

 be decided. 



The dermal scutes encircling the middle of the trunk clearly form a continuous 

 ring. None of them are sharply angulated, and all are remarkable for the coarse- 

 ness of their tubercular ornament, which can be partly seen from the inner face 

 through the frequently translucent bone. Some of these scutes are represented 

 from the inner aspect in figs. 2 b, 2 e. The anterior and posterior borders of each 

 dors< (-lateral scute (fig. 2b, d.) are nearly parallel, and the median angulation ends 

 in a sharp point anteriorly. The two truncated arms of each scute are almost 

 equal in size. The scutes of the lateral line (/.) are only about half as wide as those 

 of the dorso-lateral series, and are symmetrically triradiate, with the anterior arm 

 bluntly rounded in front, the other arms sharply truncated. The slime-canal 

 pierces the hinder half, and traverses a groove on the inner face of the anterior 

 half of each scute. Three of the scutes of the ventro-lateral series, immediately 

 behind the expanded pelvic fin-support (})lv.), are shown from the inner aspect in 

 fig. 2 c (v.). They are somewhat larger than those of the lateral line, but very 

 similar to the latter in shape, with a more sharply pointed anterior end. Their 

 postero-superior arm is a little longer than their postero-inferior arm. 



Horizon and Locality. — Zone of Micraster coranguinitm : Grays, Essex. 



