N. ROSÉN, STUDIES ON THE PLECTOGNATHS. 25 



arranged in lamellae both horisontal and vertical in direction. 

 The integumental ossifications vary rather much as regards 

 shape, in some forms (Molidae and Ostraciontidae) large plates, 

 which are connected with each other and project in long 

 spines or short tubercles, which more or less pierce the epi- 

 dermis. I consider this form to be most primitive. The next 

 stage in specialisation of the plates occurs according to me 

 in the Balistids, which possess plates with small tubercles. 

 The plates do not touch each other. Their position is not 

 horizontal, but an oblique vertical one as in the ordinary 

 Teleosts. They differ, however, from them by the absence 

 of scale-pockets. The epidermis does not project into the 

 corium. By this altered position of the plates the body is 

 not longer immovably encased in a carapace, but movable. 

 In some Balistids a reduction of the plates begins, which 

 reaches its height in the Tetrodontids and Diodontids. These 

 fishes have a comparatively minute plate, while the spine is 

 long, forming the greater part of the ossification. The shape 

 of the spines varies much as shown on the previous pages. 

 The reduction of the plates into spines is intimately con- 

 nected with the development of the air-sac. In all the ossi- 

 fications two layers can be distinguished, one superior homo- 

 geneous forming the surface of the plate and the spines and 

 tubercles projecting from it, and one inferior fibrillated. The 

 former is developed from the exteriör, the latter from the 

 interiör layer of the corium. The air-sac has caused several 

 adaptations of the skin in the Tetrodontids and Diodontids. 

 One of the most important is the development of a subcu- 

 taneous tissue which is especially rich on the ventral side in 

 those forms which are provided with an air-sac united with 

 the ventral body wall. Another important peculiarity is the 

 existence of muscles in the named two families. There is 

 one exteriör circular layer round the body and one interiör 

 longitudinal, which is developed only on the belly. 



As to the phylogenetical relation between the families 

 my studies on the integument confirm the opinion I previ- 

 ously have put forth based on the blood-vessels and the in- 

 testines. 1 The families Balistidae, Diodontidae and Tetrodon- 

 tidae form a series of evolution. The Molids and Ostraciontids 



1 See this series of Studies No. 1. The blood-vascular system (in this 

 journal Bd 7. No. 25) and No. 2. The air-sac (ibidem Bd 7. No. 30). 





