1 12 1 FOSSIL FISHES OF THE ENGLISH CHALK. 



numerous small, elongated, shallow pits at the sides. An enlarged postclavicular 

 scale above the pectoral fin. Scales large and thin, deeply overlapping ; their 

 covered portion not marked by radiating grooves, their exposed sector smooth or 

 ornamented with a minute rugosity and fine radiating ridges and furrows. 



Type Species. — Pachyrhizodus basalis, from the English Chalk. 



Remarks. — The remains of this genus were first discovered in the English 

 Chalk, and the jaws and cranium were originally described and figured by Agassiz 

 under the indefinite name Hypsodon lewesiensis (see p. 99). The description, how- 

 ever, was both inaccurate and inadequate, and part of a similar jaw was afterwards 

 referred by Agassiz himself to another genus to which he gave the MS. name 

 Pachijrhizodus. The description and figures of Pachyrhizodus basalis published by 

 Dixon in 1850, though also unsatisfactory, sufficed for recognition ; and the genus 

 Pachijrhizodus was eventually denned by Cope, who described remains of five 

 species from the Chalk of Kansas. 



The cranium of Pachyrhizodus (Text-fig. 39, p. 130) resembles that of the exist- 

 ing Ghanos, lacking the subtemporal fossa in the side of the otic region, and show- 

 ing even the same deep keel on the basioccipital. The teeth have a large expanded 

 base, which is not enamelled, and they are firmly fused with the supporting bone. 

 They are fixed in a single spaced series on a nearly flat ledge which equals their 

 basal width, and they are flanked externally by a thin low wall of bone, which is 

 often more or less destroyed in the fossils, thus giving them a longer or shorter 

 appearance when viewed from their outer side. They must have been replaced 

 alternately, each space between two teeth showing a very shallow socket (some- 

 times two shallow sockets) for a developing germ, which is rarely seen in the 

 fossils, but must have been removed with the enveloping soft tissues. 



The total number of vertebras in a skeleton of Pachyrhizodus caninus from the 

 Chalk of Kansas, in the U. S. National Museum, is between fifty and sixty. All 

 the vertebral centra are shorter than deep, and most of them have smooth sides ; 

 but some centra in the anterior abdominal region are impressed with small and 

 shallow elongated pits. There are no transverse processes, the ribs being directly 

 received in small pits on the centra. 



The small caudal region from the Chalk of Cuxton represented of one half nat. 

 size in PI. XXVI, fig. 6, may be referred to Pachyrhizodus. The vertebral centra, 

 though partly decayed, show the smoothness of their sides, and bear comparatively 

 slender neural and haemal arches. A few haamals are expanded at the base of the 

 tail, but their outlines are indistinct in the fossil. The remains of the front part 

 of the anal fin (a.) indicate that it was small and remote, with the stout rays very 

 closely articulated and divided distally. Part of the forked caudal fin (c.) displays 

 the very stout rays, which are articulated distally with close wavy sutures and 

 eventually become finely divided. The scales are large and thin, deeply overlapp- 

 ing, only marked in their exposed portion with feeble radiating furrows. 



