NILS ROSEN, STUDIES ON THE PLECTOGNATHS. 



Triacanthidae. 



I have not had an opportunity of studying the museles 

 of the forms belonging to this familv. Thilo has made a 

 few remarks on Triacanthus. The pelvic bone is not movable 

 in this genus. 



Tetrodontidae. 



Sörensen has described some of the body-muscles of a 

 species of Tetrodon, but he has not made any attempts to 

 explain the arrangement of these museles nor to compare 

 them with the museles of other fishes. Wiedersheim has 

 mentioned a transverse musclelayer inside of the skin. Thilo 

 has given some notes on the ventral museles. But no compara- 

 tive description of the body-muscles has been given. The 

 following inquiry is based on the study of Spheroides testu- 

 dineus (Fig. 3—6, 17 and 18). 



The dorsal museles show two parts, corresponding to a 

 and b of Maurer's diagram. The direction of the fibres is 

 the usual one as shown in Fig. 3. The two parts are dis- 

 tinctly separated from each other by a fascia. Supracarinal 

 museles are present both between the head and the dorsal 

 fin and behind this fin. The metamers of the dorsal museles 

 are distinct. 



The ventral museles are very interesting. A portion cor- 

 responding to vi. obliquus superior is developed. It is dis- 

 tinctly metamerical and forms the greater part of the museles 

 of the side. The direction of the fibres is the characteristic 

 one for this muscle. M . obliquus i?iferior is well developed 

 anteriorly. This part covers m. obliquus superior and is dor- 

 sally much enlarged covering also a part of the portion a of 

 the dorsal museles as shown in Fig. 3. The anterior part of 

 m. obliquus inferior is attached to the postclavicle, but some 

 fibres continue passing inside of this bone and are inserted 

 into the pectoral arch. The ventral fibres of this portion 

 are connected with m. rectus. The middle part of vi. obliquus 

 inferior has disappeared. The caudal part is present forming 

 a small area of the lateral side below vi. obliquus superior. 

 Cranially this portion continues into the exteriör medial part 



