Plagiostomi — Sharks and Rays. 



9 



have higher and sharper crowns than the former, and so likewise 

 have the detached teeth from the Lower Oolites. The dorsal spines 

 are all ornamented with longitudinal ribs, and the posterior is 

 shorter and stouter than the anterior. Orodus is an old Hybodont 

 of the Carboniferous Limestone. 



The CestraciontidtB follow the Hyhodontidm, and at least one of Table-case, 

 the genera — Acrodus — usually placed here, seems to pass almost 

 imperceptibly into the true Hybodm of the preceding family. Its 

 teeth (Fig. 20) are in most cases much less cuspidate than those of 

 the Hybodonts, but at least some species have similar dermal 

 " Sphenonchi " and ribbed dorsal spines. 



No. 29. 



Fig. 18. 



Fig. 20. 



Fig. 19. 



L 



Si 



Fig. 18.— A, spine of Lcpracanthus Colei, Owen, Coal-measures, Ruabon, N. Wales; 



B, a portion of the spine enlarged, to show the external ornamentation. 



Fig. 19.— Teeth of Acrodus Anningice. Lower Lias, Lyme Regis. 



Fig. 20.— Dorsal spine of Hybodus, Lias, Lyme Regis. 



A single genus of this family, the Cestracion or " Port- Jackson Table-case, 

 Shark " (Figs. 9, 21, 22), still survives and throws most important No. 29. 

 light upon the detached fossil teeth which are so frequently met 



