126 FOSSIL FISHES OF THE ENGLISH CHALK. 



least 30 cm. Crown of all teeth with slender apex, which is more or less curved. 

 Premaxilla scarcely more than twice as long as broad, its small marginal teeth not 

 inclined outwards, its inner teeth with the base much expanded and the crown 

 very stout and curved inwards. Dentary bone dee}), but rapidly contracting in 

 front into a narrow thickened symphysis, and sharply bent inwards in the lower 

 two .thirds throughout its length; its outer face without any deep longitudinal 

 depressions, but with a slight groove which extends backwards and upwards from 

 the middle of the symphysial border until lost at the oral border; its total number 

 of tooth-sockets between 25 and 30. 



Description of Specimens. — The type specimen is the anterior end of a right 

 dentary, originally described by Mason as the premaxillary region of a reptile. 

 Part of a similar dentary is shoAvn in association with the maxilla and premaxilla 

 in Man tell 's specimen, which was referred to Hypsodon lewesiensis by Agassiz, 

 op. <-it., pi. xxv a, fig. 2. The same jaws occur with a group of scattered head- 

 bones on the slab of chalk described by Toulmin Smith, loc. cit., 1846; and they 

 are associated with part of the cranium itself in another specimen in the British 

 Museum from Ditchling, Sussex. Although the remains are fragmentary, it is 

 therefore possible to identify many bones of this species. 



The cranium associated with jaws from Ditchling (B. M. no 49905) is so much 

 fractured that it does not exhibit the outer face of the bones ; but in size, shape, 

 and general contour it agrees so closely with the hinder half of a cranium figured 

 by Agassiz, op. cit., pi. xxv a, fig. 1, that there need be no hesitation in referring 

 the latter to the same species. It is thus clear that in Pachyrhizodus gardneri the 

 frontal bones extend backwards as far as the supraoccipital and widely separate 

 the small parietals, while a true squamosal covers the pterotic on each side. The 

 shape of the hinder part of the cranial roof resembles that of P. subulidens 

 described below (p. 129), the posterior half of the median frontal depression being 

 especially well displayed. The basioccipital bears on its lower face a deep 

 longitudinal keel, which is seen again in a fragmentary smaller specimen, probably 

 of this species, in the Toulmin Smith Collection (B. M. no. 41671). Part of an 

 ossified sclerotic is also shown in the Ditchling specimen. 



The hyomandibular is a thin lamina of bone, much expanded in its upper half, 

 with a single long articular facette at its upper end. It resembles the corres- 

 ponding element of Megalops and Ohanos in the shortness of the process for the 

 support of the operculum, and in the relative narrowness of the lower part of the 

 bone. The triangular quadrate, with very stout articular end, is well shown by 

 Agassiz, op. cit., pi. xxv b, figs. 4, 5 ; but the symplectic has not yet been 

 identified. The ectopterygoid is evidently the long thin lamina of bone which 

 occurs in several specimens covered on its inner face with a dense cluster of 

 minute, bluntly-conical teeth. A bone, which may perhaps be palatine, is partly 

 covered with a similar cluster of teeth. 



