ORDER ELASMOBRANCHEI. 



943 



Asteracanthus {Strophodus) agrees with Hybodus in the persistent 

 notochord and the presence of orbital spines, but differs in the 

 characters of the fin-spines and teeth (figs. 868, 869). The prin- 

 cipal teeth form irregular rhomboids, with slightly arched and 

 flattened crowns, marked by a reticulate ornamentation — the sym- 

 physial teeth being large, few, and simple. The fin-spines are 

 marked by stellate tubercles, which are sometimes fused, and 

 have two posterior mesial series of denticles. The type species 

 (fig. 868) is of great size, and occurs typically in the Great 

 Oolite of the Continent and England, but ranges upwards to 

 the Kimeridge Clay, and thus presents a remarkable instance 

 of persistence. This genus is also represented in the Purbeck. 

 Other extinct genera are Palceospinax, from the Lias, and Synech- 

 odus from the English Chalk, both of which have teeth closely 

 resembling those of Hybodus ; 

 but the fin-spines, at least in 

 the former, were smooth, like 

 those of Cestracion. The Cre- 

 taceous genus is the more 

 specialised, and approximates 

 in cranial structure to Noti- 

 danus, the palatopterygoid ar- 

 ticulating directly with the 

 cranium. Finally, the existing 

 genus Cestracion (in which may 

 be included forms described 

 under the names of Gyropleu- 

 rodus and Drepanophoi'us) oc- 

 curs in Europe from the Kim- 

 eridgian of Bavaria to the 



London Clay, and is now represented by four species in the 

 Australian and Japanese seas. This is a specialised genus differing 

 from Asteracanthus by the absence of orbital spines, the numerous 

 and small symphysial teeth (fig. 864), the smooth fin-spines, and the 

 full calcification of the vertebral column. 



Family Scylliid^. — This is a family of small extent, and most of 

 the members of which are of relatively small dimensions. The 

 dorsal fins have no spines, and the first is placed above or behind 

 the pelvic fin ; while the teeth are small and cuspidate, generally 

 several series being simultaneously in use. The living forms are 

 commonly known as Dog-fishes. Palceoscy Ilium, of the Lower Kim- 

 eridgian of Bavaria, seems to be allied to the existing Ginglymostoma 

 and Scy Ilium. Scylliodus, of the English Chalk, is an imperfectly 

 known form with teeth like Scyllium, but with vertebrae approximat- 

 ing to Lamna. Scyllium itself is represented in the Cretaceous of 



Fig. 869. — Lateral and oral surfaces of a tooth 

 of Asteracanthus ornatissimus, from the Corallian 

 of the Continent. (After Zittel.) 



