PACHYRHIZODUS. 131 



smooth. Nine or ten broad branchiostegal rays (br.) are fixed to the epihyal (fig. 

 1 b, eph.), while traces of those fixed to the ceratohyal show that they must have 

 been numerous. It is not known whether a gular plate was present or absent. 

 The lower segments of the two halves of one branchial arch are fused with their 

 basibranchial bone (b. a.). 



The vertebral centra appear to exhibit a greater tendency towards slight 

 pitting of the sides than those of the typical species. 



The clavicle is characterised by a very wide inwardly-directed expansion (PI. 

 XXVII, fig. 1 c) ; and to its posterior border is appended a large postclavicular 

 scale above the base of the pectoral fin. 



A few thin scales associated with the maxilla shown in PI. XXVII, fig. 3, are 

 ornamented in their exposed sector with a very fine granulation and a few shallow 

 and discontinuous radiating grooves (PI. XXVII, fig. 4). Their extensive 

 overlapped portion is smooth, without any radiating grooves. 



Remarks. — This species is intermediate between the typical Pachyrhizodus and 

 the typical Thrissopater ; and it now seems probable that the occipital part of a 

 small skull which I once referred to " Thrissopater magnus " (Catal. Foss. Fishes B. M., 

 pt. iv, 1901, p. 35, pi. v, fig. 2) should be assigned to Pachyrhizodus sabulidens. 



Horizon and, Localities. — Zone of Holaster subglobosus .• Cherry Hinton, 

 Cambridgeshire ; Burham and Hailing, Kent. Zone of Schloenbachia varians : 

 Barton, Cambridgeshire. 



5. Pachyrhizodus CO magnus, A. S. Woodward. Plate XXVIII, fig. 1. 



1901. Tlnissoimter magnus, A. S. Woodward, Catal. Foss. Fishes B. M., pt. iv, p. 34, pi. v, fig. 1. 



Type. — Imperfect head, probably from the zone of Holaster subglobosus ; British 

 Museum. 



Specific Characters. — A species of uncertain generic position, apparently con- 

 nected with Pachyrhizodus by P. subulidens, which is intermediate between this 

 and the typical species. Length of maxilla about 7"5 cm., of mandible about 

 11 cm. ; both these bones and the premaxilla resembling those of P. subulidens. 

 Teeth unusually small, but of the characteristic shape and implantation. Median 

 depression of cranial roof deep behind, but differing from that of the typical skull 

 of Pachyrhizodus in not extending further forwards than the transverse ridge 

 between the orbits. 



Description of Specimen. — This species appears to be known only by the type 

 specimen (PI. XXVIII, fig. 1), which is vertically crushed so that the cheeks are 

 displaced outwards, while the antorbital region is " faulted " by an oblique trans- 

 verse fracture. 



The most characteristic feature of the cranial roof is the median depression in 



