2 



BRITISH FOSSILS. 



the prognathic character very decided. The orbit is situated in a 

 high and very forward position. The upper limb of the operculum 

 is nearly horizontal ; the posterior margin slopes downwards and 

 forwards, in an easy curve. The preoperculum is high and narrow. 

 Wherever the surface of the cranial bones is preserved, it is covered 

 with small flattened granules, distinct from each other, and not 

 grouped in radiating or other patterns. The anterior or incisive 

 teeth are elongated cones, slightly flattened at the apices ; the 

 succeeding teeth are shorter and broader. The outer row only is 

 visible, and these being comparatively small, we may conclude from 

 the general rule applicable to this genus, that each ramus had five 

 rows of teeth, those comprised in the second row being considerably 

 larger than the remainder. One large tritor is seen in a position 

 which indicates that it belonged to the central or principal row of 

 the vomerine teeth. Several smaller ones scattered about may have 

 belonged to the lateraL series, of which there were probably two 

 on either side of the central one. The form of all these teeth is 

 more or less oblong, and the crowns are smooth, devoid alike of 

 the inequalities of the Microdon teeth, and the rugosities of the 

 Gyrodi. The dorsal and anal fins are large, and opposed to each 

 other. The former has 20 rays, the latter about 15. These are all 

 articulated to dilated extremities of strong interapophyseal osselets. 

 The ,Tays are single at first, but they soon dichotomize, and are 

 marked by very closely arranged transverse articulations. The 

 course of the spinal column is nearly straight, and does not coincide 

 with the arched form of the dorsal outline. The vertebral centra 

 were cartilaginous, but the apophyses strong and bony. The scales 

 are much mutilated. Those of the nuchal region are the largest. 

 The impression left on the stone proves them to have been orna- 

 mented with diverging curved lines of sm.all tubercles, sweeping 

 downwards and outwards from the central area of each scale. The 

 succeeding scales below are of smaller dimensions, but they all 

 partake of the high narrow character so generally prevailing in this 

 family. They are covered with small flattened tubercles, similar to 

 those found on the bones of the head. No evidence remains of the 

 form or position of the pectoral or ventral fins. It has been stated 

 above that the form of this fish differs from that recognized as 

 typical of the genus Pycnodus, but this remark must be taken as 

 having reference to the state of our knowledge of the genus. 

 Agassiz founded its generic attributes on the well-known Pycnodus 

 platessus, of the Monte Bolca beds, and in this tertiary form the 

 elongation of the pedicle of the tail is very striking. The only 



