1 26 PALEONTOLOGY 



all existing fishes, significantly testifies to a principle of " pro- 

 gression." 



The external " ganoid " surface of the buckler-plates of 

 Coccosteus is ornamented with small hemispherical tubercles ; 

 whence the generic name, signifying " berry-bone." The 

 similarity of this ornamentation to that of the plates of the 

 buckler in some tortoises, led to the belief, when the coccos- 

 teal plates were first found, of their being evidence of the 

 chelonian genus Trionyx in Devonian beds. Passing notions 

 also got into print of the crustaceous affinities of Coccosteus ; 

 whence the trivial name of the type-species decipiens, or 

 the " deceiving " Coccosteus. . 



Strange as seem the forms and structure of the placo- 

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

 existing species which throw much truer light on their nature 

 than any existing Chelonia or Crustacea. The singular little 

 family of " trunk-fishes " (Ostracionida;) shows 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 Cephalaspis. The caudal part of the trunk 

 protrudes from the back opening of the cuirass, as in Coccos- 

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

 incomplete. 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 Gallich- 

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



* Histoire des Poissons, torn. xii. 



