OF ARTS AND SCIENCES. 273 



and are adapted for special uses, as in the young of some Gadoids, or 

 those genera in which the rays of the ventrals extend into large fila- 

 ments, which may be of use as tactile organs. The most charac- 

 teristic of these genera are found among some of the newly discovered 

 deep-sea Fishes dredged by the " Challenger " and by the " Blake." 



In the Fishes living at moderate depths and in pelagic Fishes the 

 pectorals or ventrals may be developed into organs of flight, as we 

 find it to be the case in the young of Onus, which certainly mimics to 

 an extraordinary degree in its embryonic stages the Flying-Fishes. 

 The specialized ventrals of the embryonic stages of Lophius and Onus 

 may represent the huge ventral appendages, articulated fins, which 

 exist in Pterichthys and other Devonian Fishes. The absence of 

 ventrals or the presence of small ventrals and the existence of a large 

 anal fin, still more or less united with the caudal and dorsal fin, may 

 thus be regarded as embryonic characters. The differentiation of the 

 anal is the next stage of development, and well-developed, isolated 

 anals and ventrals are generally found to occur with well-developed 

 and isolated dorsals. The existence of abnormally developed ven- 

 trals, as in young Gadoids, may also be considered as an embryonic 

 character. 



As far as the oldest fishes are concerned, we find in them the same 

 dorsals and anals isolated from the heterocercal tail fin, just as they 

 exist in many of the Fishes of the present day, and there is nothing to 

 show that in the earliest known fossil Fishes the development of the 

 median fins did not take place much in the same manner as it takes 

 place to-day in the young of Lepidosteus. 



There is something in the general structure of the youngest embryos 

 of Lumpus which recalls to us the Cephalaspidse. The position of 

 the mouth in all young bony Fishes is characteristic of the earliest 

 Fishes ; they have in common also a cartilaginous skeleton, heterocercal 

 tails, and a rudimentary dorsal and anal, with prominent pectorals, as 

 in some of the fossil genera. With the Dipteridae, although we have 

 median fins broken up into several distinct fins and a heterocercal tail, 

 yet these fins all belong to the embryonic posterior dorsal and anal. 

 In the next prominent group, the Acanthodida, the heterocercal tail 

 continues and is found to exist with single anal and dorsals, and small 

 ventrals with well -developed pectorals. While in the Palseoniscidjie, 

 the Dapedidse and Pycnodonts, we find the representatives of embry- 

 onic types in which the tail becomes much less heterocercal, the anals 

 and dorsals are each one long continuous fin with numerous rays, recal- 



VOL. XVII. (n. S. IX.) 18 



