FISHES. 279 



is found in the eocene of Sheppey ; and Chondrosteus is found 

 in the lias of Lyme Regis. 



2nd Family. The SYI* G^ATHID^;, of which Syngnatlius is 

 one of the types; they are encased in a cuirass of bony plates, 

 of a form which gives the body an angular figure. This 

 family includes many living, and two fossil genera from 

 the beds of Monte-Bolca. 



3rd Family. The DIODONTIDJE Have the jaws covered 

 with enamel, which represent the teeth. Their scales 

 develope long spines. We know many living genera and 

 a single fossil Diodon from the tertiary strata. 



FIG. 190. Ostracion quadricornis. 



4th Family. The OSTEACIOKID^!, of which the Trunk- 

 fish Ostracion (fig. 190) is typical. The body is encased in 

 an osseous box, formed of polygonal plates. Fossil genera of 

 this family are found in the cretaceous and tertiary strata. 



5th Family. The CEPHALASPIDJG have the head and 

 the anterior part of the body covered with osseous plates, 

 which sometimes form a complicated cuirass. They have a 

 heterocercal tail, or that organ is absent ; the head is flat 

 and round ; the mouth is situated at the under surface 01 

 the head, and is often without teeth ; the body is flat ; the 

 pectoral fins are sometimes greatly elongated in some genera 

 and absent in others ; the ventrals are always deficient ; the 

 skeleton is very simple' and reduced to a rudimentary deve- 

 lopment of its peripheral parts ; the dorsal cord is persistent 

 through life, to which are attached bony processes. It is 

 remarkable to find in the adult condition of this ancient 

 family several of the phases of structure which all fishes 

 exhibit in the course of their embryonic development. The 

 several genera of which it is composed have been a great 

 puzzle to palaeontologists ; their remains, when first dis- 



