2 ARKIV FÖR ZOOLOGI. BAND 8. NIO 18. 



ently taken into consideration by Maurer. In portion a of 

 the ventral muscles the muscle-fibres run horizontal or in a 

 direction from caudally-ventrally to cranially-dorsally. The 

 muscle of this portion is called m. obliquus superior. Portion 

 b is not uniform. The line bounding this portion does not 

 run straight, butshows slight angles. Maurer distinguishes three 

 parts of this portion, called a, [B and 7. The muscle-fibres 

 of portion a have generally a horizontal direction (ra. obliquus 

 medius), those of portion (3 a direction from caudally-dorsally 

 to cranially-ventrally (ra. obliquus inferior). In the portion 

 7 as well as in the two portions c and d the muscle-fibres 

 are horizontal (ra. rectus). Already in the Selachian group a 

 reduction of some portions of the ventral muscles begins. In 

 the Teleosts ra. obliquus medius has disappeared. No true 

 rectus in developed. A rectus is still formed by the ventral 

 portions of the other muscles by a change of the direction 

 of the fibres. Maurer has found, that ra. obliquus superior 

 and ra. obliquus inferior are rather differently developed in 

 the Teleosts. In some forms ra. obl. sup. is very large form- 

 ing the greater part of the body muscles and covering the 

 small ra. obliquus inf., in others ra. obl. inf. is well developed 

 covering ra. obl. sup. Both muscles can take part in forming 

 a recto-muscle. A portion of this muscle is attached to the 

 anal and pelvic fins (ra. infracarinales of the descriptive litte- 

 rature). Maurer has, as above mentioned, studied the dorsal 

 muscles comparatively little. These muscles do not seem 

 to vary so much as the ventral ones. As my studies con- 

 cern only the Plectognaths, it is of course very difficult to 

 make comparisons with other forms as regards this part of 

 the body-muscles. 



I intend in the present paper to deal with the body- 

 muscles. The forms I have had an opportunity of studying 

 are Mola, Balistes, Spheroides and Lactophrys. I shall also 

 give some notes on the muscles of the unpaired fins. Very 

 little has hitherto been written on the body-muscles of the 

 Plectognaths. Cuvier and Meckel have given a few notes 

 thereon. Sörensen and Wiedersheim have described some 

 of the muscles of the Tetrodontids, Thilo a few muscles of 

 the Balistids and the Tetrodontids, Clelanp, Wahlgren and 

 other authors have studied the Molids. 



