22 ARKIV FÖR ZOOLOGI. BAND 10. NiO 8. 



the Balistids, is wedged in between the walls of a fur- 

 row of squamosum. A postcJavicle is present. In Balistes 

 this bone takes part in forming a soundirig apparatus. Mö- 

 Bius writes (p. 1002) about it as follows: »Der Schall ent- 

 steht durch Schwingungen des oberen Hebelarmes des Post- 

 claviculare, wenn dessen längerer unterer Hebelarm durch 

 den unteren Seitenrumpfmuskel schnell riickwärts gezogen 

 wird. Er wird verstärkt durch tJbertragung der Schwin- 

 gungen auf die Clavicula und auf die Wand und Luft der 

 Schwimmblase, und diese pflanzt ihn durch die beiderseitigen 

 beweghchen Hautplatten in das umgebende Medium fort.» 

 Pelvic hones and ventral fins are absent in the famiHes Moli- 

 dae, Oslraciontidae, Tetrodontidae and Diodontidae. In the 

 other families a single pelvic bone is developed. It is firmly 

 united to the pectoral arch in Triacanthiis, but movable in 

 Triodon and in the Balistids. The ventral fins, if present, 

 are reduced to a short single spine attached to the end of 

 the pelvis. The mobility of the pelvic bone, and also its 

 absence in the Diodontids and the Tetrodontids, are closely 

 related to the development of the air-sac, as Thilo has 

 shovvn clearly in his interesting treatises (ef. this series of 

 studies. No. 2. The Air-sac etc. Arkiv f. zoologi Bd. 7. 1912). 



D. Median Fins. 



Bridge deals in his treatise on the mesial fins with Bali- 

 stes, Monacanthus, Tetrodon, Diodon and Mola. The radial 

 elements are bisegmental, with proximal and distal segments. 



Balistidae. 



The anterior dorsal fin is highly specialized. The fused 

 radial elements form a curious boat-like structure, which 

 rests anteriorly on the skull. This apparatus has been describ- 

 ed in details by Sörensen, to whose paper I refer. The 

 proximal segments of the posterior dorsal and anal fins ter- 

 minate, as stated by Brigde, in cartilaginous extremities, 

 which form a »dorsal margin traversed by a slight longitu- 

 dinal groove for articulation with the series of distal seg- 



