228 



FOSSIL FISHES OF THE ENGLISH CHALK. 



before or during fossilisation. One such specimen in the Brighton Museum has 

 been described under the name of Aulodus agassizi} 



The vertebral centra have hitherto been found in direct association with the 

 teeth of Ptychodus only in one specimen of P. decurrens, now in the British 

 Museum (PI. LII, fig. 6). They are biconcave discs of approximately circular 

 shape, remarkable for their very short antero-posterior measurement. The crushed 

 and broken example shown in PL LII, fig. 6, exhibits a fragment of the nearly 



Fig. 72. Ptychodus mammillaris, Agassiz ; vertical transverse section (A) and horizontal section (B) of tooth, 



highly magnified. After Agassiz. 



smooth outer face of the primitive double-cone (a.), and also a portion of its 

 inner layer (/>.), which is marked by numerous fine radiating ridges. Within the 

 double-cone the centrum is strengthened by closely arranged concentric laminae 

 (c), which are so delicate that they are often distorted or even partially destroyed 

 in the fossils (PI. LII, fig. 16). These laminae are pierced by a few rounded pores 

 in irregular order, but they are never united by cross-bars. The thin middle part 

 of the centrum is solid (seen in fig. 10, but broken away in fig. 6). Where each 

 1 F. Dixon, Geol. Sussex (1850), p. 366, pi. xxxii, fig. 6. See also so-called " incipient teeth of 

 Ptychodus" op. cit., pi. xxx, figs. 4, 5. 



