N. ROSÉN, STUDIES ON THE PLECTOGNATHS. 26 



given. It is to be remarked that the distal segment are 

 elongated. The extremities of the proximal elements have 

 fused to form a large cartilaginous piece, which is traversed 

 by a longitudinal groove with which the distal segments arti- 

 culate. The fins are movable exclusively from side to side. 



Ostraciontidae. 



I have examined Lactophrys trigonus. The distal seg- 

 ments are small. Their ventral ends are covered with fi- 

 brous cartilage, which forms a joint with the fused extre- 

 mities of the proximal segments. 



General Remarks on the Fins. 



It is characteristic of the posterior dorsal and anal fins 

 of the Plectognaths that they are movable mainly from side 

 to side. They can not be depressed. The »pelvic» bones 

 are reduced to single vertical piece or have entirely disap- 

 peared. The ventrals are absent or form a single spine. The 

 anterior dorsal fin does not show any peculiarities in Tria- 

 canthus, but is highly modified in the Balistids. In the other 

 forms this fin is absent, The superior supracleithral is firmly 

 united to the skull. 



Arkiv föl- zoologi. Band 10. N:o 8. 2* 



