PLAGIOSTOMI 



105 



the " lias," which is the oldest or lowest of the great " oolitic " 



system, the dorsal spines of the genus Hyhodus (fig. 26), are 



the largest and most abundant ; this genus, however, is 



represented by detached teeth in the keuper and 



muschelkalk members of the " trias." The lias 



formations give evidence that the dorsal spines 



and fins of Hyhodus were two in number ; and 



the genus is shown, both by the structure of 



the spine and the form of the teeth, to have had 



its nearest affinities with the Cestracion amongst 



existing Plagiostomes. Hyhodus continued to 



be represented by successive and varying specific 



forms up to, and including, the cretaceous period. 



Hyhodus is therefore a genus of cartilaginous 



fishes eminently characteristic of the secondary 



or mezozoic period in palaeontology, and ranges 



through every formation of that period. The 



specimen selected for the illustration of the 



dorsal spine of Hyhodus is that of the H. suh- 



carinatus, from the Wealden of Tilgate Forest. 



Large fossil spines, longitudinally grooved, 

 have been found associated with the teeth of the 

 extinct cestraciont genus (Ptychodus) of the 

 chalk formations. 



In the tertiary formations, the fossil spines 

 present for the most part the generic characters 

 of those of existing Plagiostomes — e, g., Spinax, 

 Trigon, and Myliohates ; but one form, found in the eocene 

 beds near Paris, is the type of the extinct genus Aidacanthus 

 of Agassiz. 



The teeth of the plagiostomous fishes — viz., sharks (Squa- 

 lidce), rays (Raiidce), and Cestracionts, are very numerous, 

 and, being attached only by ligament to the membrane of 

 the mouth, they must soon fall off in the decomposition of 



Fig. 26. 



Hyhodus sub- 

 carinatus. 

 (Wealden.) 



