Actlnopterygii — Flectognathi. 121 



fishes {e.g., Ostracion) have the integument converted into a 

 continuous mosaic of hexagonal scutes ; in others (e.g. Biodon), 

 the skin is covered with numerous isolated spiny ossifications ; 

 while others are almost destitute of hard dermal structures. 

 The pelvic fins are either absent or merely represented by 

 spines ; and the bones of the upper jaw are nearly always 

 firmly united. 



Fig. 165.— Teeth of Diodon Scilhe, Ag. ; Miocene, Malta. 



The sub-order is divided into the two families of Sclerodermi 

 and Gymnodontes, the former of which have the jaws armed 

 with distinct teeth, Avhile the latter ai-e provided with a kind of 

 edentulous beak. Fossil Scleroderms, in an excellent state of Table-case, 

 preservation, are found in the Eocene Slates of Glaris, where 

 two extinct genera, Acauthoderma and AcantJwpleurns, occur. 

 A species of Ostracion has been described from the Eocene of 

 Monte Bolca ; and Glyptocephahos appears to be another repre- 

 sentative of the family in the London Clay of Sheppey. The 

 Gymnodonts also date back to Eocene times. Fine specimens 

 of Diodon (" Sea-hedgehogs") are exhibited from Monte Bolca 

 and the Infusorial Earth of Oran, Algeria ; and the little piles 

 of dental plates (Fig. 165), placed within the mouth of the 

 same genus, are found in the Miocene of Malta, Sicily, and 

 other localities. Enneodoii is a closely allied extinct genus, 

 from the Middle Eocene of Monte Postale, N. Italy. 



9 8 



