Nr. 9] CONTRIB. TO THE LIFE HISTORY OE THE FISHES 33 



tioned species of shark. Wyman has, al a certain slage in the 

 embryonal development ol" Raja batis, remarked the presence 

 of an anal fin; as a matter of fact two such (vide Lutken, 1. c. 

 p. 269). In such stages of R. radiata as I have had at my 

 disposal, there was only 1 anal fin, which is perhaps most 

 distinctly prominent in the 60 mm. slage (fig. 9) hut which 

 becomes reduced during the subsequent capsule slages, until at 

 last the fold of skin representing the anal fin presents the ap 

 pearance of a dermal tube, situated in the median line on the 

 underside of the lail. The presence of this anal fin is one of 

 the points which serve to distinguish the ravs as an adaption 

 from the shark type. The anal fin is, for practical reasons, sub 

 sequently discarded, as the ravs, living as they do on the bot- 

 tom, would have no use for such; on the contrary it would 

 rather be a disadvanlage. Beside the features already men 

 lioned, the free flaps of Ihe pectoral fins, and the anal tin, 

 the development of the rays is, like that of the sharks, cha 

 racterised by Ihe existence of provisorv gill threads. In R. 

 radiata, these commence as small knobs in the upper edge of 

 the gill slits, some being visible already at the 18 mm. stage. 

 It is worth noting, that although the first formation of the 

 spiraculum proceeds in a manner similar to that of the gill 

 slits, there is vel no gill thread in this, thus indicating a func 

 tional difference between gill slits and spiraculum. The gill 

 threads disappear, in the case of R. radiata, when the develop 

 ment of the embryo is so far advanced that the dorsal side has 

 attained pigmentation. 



Special mention should be made of the comparatively long 

 tail end behind the dorsal fins. as this is not found in R. radiata 

 after the young have emerged from the egg capsule. The Swedish 

 naturalist A. W. Malm (Bidrag till Kånnedomen om Utvecklingen 

 av Raja; Ofvers. Kgl. Vet. Akad. Forh. 1867, Nr. 3, p. 91—102, 

 tab. III, figs. 1 — 4) noticed Ihal the dorsal fins are formed fairly 

 far forward on the tail of R. clavata, and he supposed thai 

 these fins disappeared in the course of the embryonal develop 

 ment, the fins subsequently formed being situated nearer the point 

 of the tail. This suggestion, in it seif soinewhat unreasonable, 

 was contradicted by Ad. S. Jensen 1 , who found, in an embryo 

 of R, radiata from Greenland, that the lail end was also present 

 in the later part of the capsule period. Jensen therefore sup 

 posed that the dorsal fins mainlained their original position, 

 and that it must be the tail end which was discarded. My 

 observations here recorded serve to confirm Jensens supposition. 



1 The Selachians of Greenland; Særtr. av Mindeskrift tor Japetus 

 Steenstrup, Copenhagen, 1914. 



3 



