1^10.] CAUDAL FIN OF THE TELEOSTOMI. 599 



PL0T03US sp. ? (Silui-iiho.) (Pliite XLVII. fig. 7.) 



The caudal fin of Plotosus is very interesting and very simple ; 

 it forms part of a continuous median fin and terminates a graduiilly 

 t-ipering tail. All the specimens examined were from Torres 

 Straits and were presented to me by the late Professor Bridge. 

 They were all young, but the skeletal elements were quite fully 

 established. Before entering into a description of the iin, it will 

 be instructive to call attention to the remai'ks made by Mr. Tate 

 Regan before the Zoological (Society on April 28th, 1908, con- 

 cerning the caudal tin of C tiidoglanis inegastoini, also a member of 

 the Plotosin;^. He writes : — - 



" The Siluroid fishes of the subfamily Plotosinje have been 

 generally defined as having two dorsal fins, a short anterior one and 

 a larger posterior fin continuous with the caudal, similar to the 

 long anal. The so-called second dorsal proves to be a procurrent 

 portion of the caudal fin which has extended forward along the 

 back, and, in the species exhibited, has a base of more than two- 

 thirds the total length of tlie fish, and is formed of about 130 

 rays ; these are supported directly by the neural spines, in striking 

 contrast to the externally similar anal fin, the rays of which ai-e 

 attached to a series of interhfemals." Presumably inteilaa^mals 

 are synonymous with radials. 



On examining Plotosus, winch is a very close relative, I was 

 pleased to find that the same characters were exhibitetl in both 

 cases, or, I believe, practically the same, for although in Plotosus 

 the rays of this "procurrent caudal" are not actually attached to 

 the neural spines but interdigitate between them, they have no 

 other endoskeletal support, as the anal rays have. 



Now, the conclusion to be dl-awn from the above quotation is 

 that Mr. Regan would define a caudal fin as that part of the 

 median fin system which is directly supported by hsemal or neural 

 spines, or in the language adopted in this j^aper, by hypurals and 

 epurals alone. It must be pointed out, however, that fin-rays of 

 true caudal fins are frequently supported by undoubted radials, 

 in addition to hypurals and epurals, and intei'digitating between 

 these supports ; throughout the GadidaB this is the case, many 

 Acantho2)ter3^gii show the same thing, as also does Clarias just 

 considered. But I fully agree with Regan's remark that the 

 so-called second doi'sal fin is part of the caudal, but for an entirely 

 different reason. I hope to deal with the definition of the caudal 

 fin at a future time after my observations have been more extended ; 

 but for the present, in connection with this discussion I base my 

 agreement witli Regan's conclusion on the natui'C of the fin -rays 

 themselves, and not on their endoskeletal supports. The dermo- 

 trichia of the caudal fin difier from those of the remainder of 

 median fins in the method of attachment to their internal 

 supports ; the caudal fin-rays of Teleosts attach themselves by 

 means of a forking at tlieir proximal ends, while those of the 

 dorsal and anal fins ai-e provided with a kind of socket by which 

 they are attached to the distal segment of the radial. So far as 



