1 48 PALEONTOLOGY. 



ganoid fishes of the " old red" period, there are not wanting 

 existing species which throw mnch truer light on their nature 

 than any existing Chelonia or Crustacea. The singular little 

 family of " trunk-fishes (Ostracionidce) shews species in which 

 the body is inclosed in a more or less quadrangular cuirass, 

 composed of suturally- articulated ganoid plates, which are 

 usually tuberculated on the external surface, and with the 

 angles prolonged into spines in some species, like those of the 

 helmet of C&plialaspis. The caudal part of the trunk pro- 

 trudes from the back opening of the cuirass, as in Coccosteus 

 and Pterichthys, and ossification of the endo-skeleton is incom- 

 plete. The species of this family are for the most part natives 

 of seas of tropical or warm temperate latitudes. 



In another family of existing fishes, called "Siluroids," 

 there are species in which the broad cranial bones, connate 

 with dermal ossifications, form a helmet to the head, whilst 

 one or two dermal spine-bearing bones combine to form the 

 part called " buckler" by Cuvier* In the genus Doras, the 

 lateral line is armed with bony ganoid plates ; and in Callich- 

 thys, these biserial piates are developed so as to incase the 

 whole body. But generally, as in Pimeloclus, the hinder 

 muscular part of the trunk is undefended, as in Coccosteus. 

 The ganoid plates of the head and back shields are fretted 

 with rows or ridges of confluent tubercles, radiating from the 

 centre to the circumference of the plate, whilst the inner 

 surface is smooth, as in Coccosteus (fig. 62); and, moreover, 

 the dorsal plate in existing Siluroids sends down a median 

 ridge from its inner surface, like that from the " dorso-median" 

 plate in Coccosteus. The point of resemblance to be mainly 

 noticed, however, is the contrast furnished by the powerful 

 armature of the head and back with the unprotected naked- 

 ness of the posterior portions of the creature — a point specially 

 noticeable in Coccosteus, and apparent also, though in a lesser 



* Histoire des Poissons, torn. xii. 



