MALACOPTERI. 175 



adequate grounds for determining the general character and 

 grade of the vertebrate animals that swarmed in the seas 

 precipitating such deposits. 



We cannot, from present knowledge, assign to any past 

 period of the earth's history a characteristic derived from a 

 fuller and more varied development of the entire class of 

 fishes than has since been manifested, nor predicate of the 

 present state of the class that it has degenerated in regard 

 either to the number, bulk, powers, or range of modifications 

 of the piscine type. A retrospect of the genetic history of 

 fishes imparts an idea rather of mutation than of progression, 

 to which the class has been subject in the course of geolo- 

 gical time. Certain groups, now on the wane, have formerly 

 existed in plenary development ; as, e. g., the ganoid order in 

 the mezozoic period, and the cestraciont form of Plagiostomes 

 in both palaeozoic and mezozoic times. 



As to the variety of the forms of fishes, seeing that the 

 earth yields no indisputable evidence of Ctenoids or Cycloids 

 anterior to the cretaceous epoch, yet still retains living repre- 

 sentatives of both Ganoids and Placoids, the present might 

 appear to be the culminating period in the development of 

 fishes, in respect of the number of ordinal forms or modifica- 

 tions of the class. It represents, however, rather the results 

 of mutation, depending upon the progressive assumption of a 

 more special type, and the Scomberoids seem now to be at the 

 head of the piscine modification of the vertebrate series. But 

 as the retention of general vertebrate characters, in the earlier 

 forms of fishes, implies closer affinity with the air-breathing 

 cold-blooded class, so a higher character of organization may 

 be predicated of the palaeozoic Placoids and Ganoids than of 

 the Ctenoids and Cycloids forming the great bulk of the class 

 at the present day. The comparative anatomist dissecting a 

 shark, a Polypterus, or a Lepidosteus, would point to the 

 structures of the brain, heart, generative organs, and in the last 



