NILS ROSÉN, STUDIES ON THE PLECTOGNATHS. 11 



sents a small remnant of this muscle is probable. As regards 

 the muscles the tendons of which are attached to the dorsal 

 and anal fins, I consider them to be profound muscles of these 

 fins. The profound muscles consist of two distinct muscles, 

 m. erector and m. depressor. In Mola they have coalesced 

 with each other. Only one tendon is present. The muscles 

 inserting into the rays of the caudal fin correspond to the 

 profound muscle of this fin or represent the caudal portion 

 of the profound muscles of the dorsal and anal fins, if the 

 true caudal fin is reduced and the dorsal and anal fins have 

 been united, as supposed by some authors. This view of the 

 nature of the muscles of Mola is suppor ted by the arrange- 

 ment of the muscles in Lactophrys, described above. In this 

 form the profound muscles of the dorsal and anal fins are 

 very enlarged, especially those of the dorsal fin. In Mola the 

 profound muscles have considerably increased. The body- 

 muscles have quite disappeared in the dorsal region. Of the 

 ventral body-muscles only two small portions have persisted 

 in the wall of the body-cavity, the whole caudal portion has 

 disappeared. The small muscles between the head and the 

 dorsal fin are reduced supracarinal muscles. 



The reduction of the body-muscles of Mola is not sur- 

 prising. The corium of this fish is very härd and tbick. It 

 forms quite a carapace. The body is immovable. Cleland 

 has shown the importance of the dorsal and anal fins in 

 swimming. The tips of the pterygiophores have coalesced 

 into one cartilaginous piece which is provided with a longi- 

 tudinal groove, in which the rays of the fins move (Fig. 11). 

 Cleland has, however, made a mistake about the fin rays. 

 They are not, as he believed, united to each other along the 

 base. Each ray is free, as seen in Fig. 13. The dorsal and 

 anal fins are movable from side to side, but all rays are 

 not necessarily moved at the same time. The flexion of the 

 fins is waving. Cleland has described the movement of the 

 fins in this manner, but I think that such a movement would 

 be very slight, if the basal parts of the rays were united, 

 as he has described them. There is another factor to take 

 into consideration for a correct understanding of the swim- 

 ming movements of Mola, but which has been quite over- 

 looked. The caudal fin is movable. There is a distinct 

 joint between the base of the rays and the pterygiophores 



