E. T. NEWTON ON FISHES' TAILS. 87 



heterocercal. The monkfish (Squatina) and many other Elasmo- 

 branchii are more diphycercal than heterocercal." 



Among the Elasmobranchs therefore, we have forms of tails 

 varying from stage 1 to stage 3 of the flounder. It is worthy of 

 remark that in this group we never find the homocercal form, for 

 to whatever extent the end of the column may be bent up, hypural 

 plates are never developed. 



5. Ganoidei. — The recent Ganoids may be divided into two 

 groups, the one including the sturgeon (Acipenser), Spatularia, 

 and Scapirliynclius, the second including Lepidosteus, Potypterus, 

 and Amia. 



(a) In the first group the tails are all heterocercal, the per- 

 sistent notochord extending into the fleshy upwardly directed 

 termination of the body, while the tail-fin is placed almost wholly 

 below it. The tails therefore of these are like the heterocercal 

 Elasmobranchs, and agree with the heterocercal condition of the 

 flounder embryo. Stage 3. 



(b) In the second group of Ganoids we find great diversity of 

 tail development. In all the recent forms the vertebral column 

 consists of well-developed bone, and it is only the extreme 

 end of the notochord which remains unossified. Polypterus 

 has the tail externally as nearly as possible alike above and below 

 the middle line, and is not unlike the tail of the Ceratodus. In- 

 ternally, however, it is found that the vertebras become smaller and 

 smaller, and terminate in a cartilaginous style slightly turned up- 

 wards, with the extremity turned down again. One or two some- 

 what enlarged haemal spines represent hypural bones, and the fin 

 rays are arranged nearly equally above and below ; but on counting 

 them it is found that while there are only 8 fin rays above, there 

 are 14 below the termination of the notochord. This tail therefore 

 may be regarded as between the diphycercal and the heterocercal 

 forms, or it may be a homocercal of the cod-fish type, representing in 

 a well-ossified condition, the earliest stage of the upward turning of 

 the tail in the embryo flounder. Lepidosteus has its tail ex- 

 ternally distinctly heterocercal, although not so markedly so as is 

 the shark's and sturgeon's. The vertebral column, which is well 

 ossified, passes gradually upwards, the vertebras decreasing in size, 

 and terminates in a long cartilaginous style in the upper edge of 

 the tail fin. The tail, therefore, is wholly below the vertebral 

 column, and is supported by numerous inferior vertebral arches (14 



