PACHYRHIZODUS. 123 



fig. 5 b) is ornamented with coarse, rugose markings, among which ridges radiating 

 from the point of suspension are conspicuous. The preoperculum (fig. 4, pop.) 

 must have had a triangular expansion at its lower end. So far as preserved in 

 the type specimen, the branchiostegal rays (br.) are nearly smooth, but part of the 

 large gular plate (<ji(.) exhibits a slight rugosity. 



The vertebral column is only partially exposed in the type specimen, but the 

 centra throughout the abdominal region are not longer than deep, and they are 

 clearly strengthened by numerous thin longitudinal ridges. 



The clavicle (fig. 4, cl.) and the very large supraclavicle (sol.) are nearly 

 smooth, and there seems to be a small smooth postclavicular plate (pel.), which is 

 overlapped and deeper than wide. The pectoral fin (pet.) comprises at least 

 fourteen rays, of which the articulated distal ends are destroyed in the fossil. 

 The long non-articulated bases of some pelvic fin-rays (plv.) are seen below the 

 dorsal fin, which is represented only by some fragments of extremely stout 

 rays (do.). 



The scales are large, thin, and deeply overlapping. Most of them are smooth 

 and exhibit only concentric lines of growth, but there are occasional traces of an 

 outer layer of very fine granulations. The course of the lateral line is marked by 

 a smooth flattened ridge. 



Horizon and Localities. — Zone of Holaster subglobosus : Kent; Dorking, Surrey. 



Genus PACHYRHIZODUS, Dixon. 



Megalodon, L. Agassiz (non Sowerby, 1829), Poiss. Foss., Feuill., 1835, p. 55 (in part). 

 Hypsudon, L. Agassiz, loc. cit., 1837, p. 104 (in part). 

 Rapliiosaiirus, R. Owen, Trans. Geol. Soc. [2] vol. vi, 1842, p. 413. 

 Pachyrhizodus, F. Dixon (ex Agassiz MS.), Geol. Sussex, 1850, p. 374. 

 Acrodoutosaurus, J. W. Mason, Quart. Journ. Geol. Soc, vol. xxv, 1869, p. 444. 



Generic Characters. — Parietal bones small and separated by the supraoccipital ; 

 frontal region indented with a deep and wide median depression, which is crossed 

 by a slight transverse ridge between the orbits and becomes deepest in front of 

 this ridge. External bones smooth or exhibiting only fibrous texture. Teeth 

 robust and conical, with a small pulp-cavity, fused at their expanded non-enamelled 

 base with the supporting bone ; mandible with a single close series nearly regular 

 in size ; premaxilla horizontally extended, with a single close series of small teeth 

 round the antero -lateral border and two larger teeth (or one tooth and its successor) 

 within ; maxilla robust, with a single series of teeth smaller than those of the 

 mandible. Minute conical teeth clustered on some bones of the roof of the mouth. 

 Vertebral centra deeper than long, slightly constricted, smooth or impressed with 



