NILS ROSÉN, STUDIES ON THE PLECTOGNATHS. 7 



cluding the head and the greater part of the tail (Fig. 6 and 

 17). Wiedersheim has noticed and figured the part covering 

 the head. To get a clear view of the development of this 

 muscle layer, I think an ontogenetical study is necessary, 

 but it seems to me rather probable that the transverse muscle 

 I have described in Balistes passing from the postclavicle to 

 the pelvic bone is the beginning of the muscle present in Sphe- 

 roides. In Spheroides the pelvic bone has quite disappeared. 

 The belly is inflatable, as I have described in a previous 

 paper. Perhaps, this transverse muscle, which is derived 

 from ra. obliquus inf., has become enlarged. Whether the 

 other body muscles have contributed to its building is, I 

 think, quite impossible to judge without an ontogenetical 

 study. The young specimens of Spheroides which I have had 

 at my disposal show the muscle quite developed and give 

 no hints on the solution of this problem. That this trans- 

 verse muscle as well as the rectus muscle are of great im- 

 portance for the emptying of the air-sac I have shown in my 

 paper on this organ. The structure of the ventral body- 

 musles have been strongly influenced by the development of 

 the air-sac. Infracarinal muscles are present behind the 

 anal fin. 



The dorsal fin is not provided with superficial muscles, 

 but with profound ones, which are not of any special inter- 

 est. The anal fin has one large superficial inclinator muscle 

 and one very small situated caudally to this. The profound 

 muscles are likewise present. 



Ostraciontidae. 



The body of these fishes have been mentioned in just a 

 few words by Cuvier 1 and Meckel 2 . Both authors point 

 out, that the lateral muscles are not attached to the ver- 

 tebral column. This is not quite correct, as shall be shown 

 below. My studies are based on specimens of Lactophrys 

 trigonus L. (Fig. 7 — 9). 



The dorsal and ventral muscles are well separated from 

 each other. The portion a of the dorsal muscles forms a 



1 Lejons, publ. par Duméril. 2. Ed. (1833). T. I, p. 304. 



2 Syst. d. vergl. Anat. T. 3. p. 74. 



