N. ROSÉN, STUDIES ON THE PLECTOGNATHS. 21 



ahsent in all. The ethmoid hones and the parasphenoid are 

 extraordinarily well developed. Parasphenoid has a high com- 

 pressed shape. Maxillare is jirmly united to praemaxillare ; 

 in adult speciinens of some förans jused with it, ivhich has given 

 the group its name. The jatvs are very small. Palatinum is 

 small. The opercnlar hones are small. Interopercidiim is in 

 most species rod-shaped, forming a part of a ligament. Hyo- 

 mandibulare and symplecticuni do not reach each other. The 

 lateral occipitals are large, reaching each other in the dorsal me- 

 dian line. The frontals are paired. On the other hand we 

 find some characters showing a specialization. In Balistes the 

 supraoccipitale has been removed cranially, which is caused 

 by the cranial displacement of the anterior dorsal fin in con- 

 nection with the peculiar function of the first spine of this 

 fin. In Monacanthus the displacement is more cranial. The 

 anterior dorsal fin, however, has, disappeared intheDiodon- 

 tids and Tetrodontids, in which forms we find the supraocci- 

 pitale rather little, if at all, removed from its general place 

 in the teleostean skull. A redisplacement has without doubt 

 taken place, as we have every reason to believe that these 

 groups form a continuation of the line of development, which 

 begins with the Balistids. In order to give a quite charac- 

 teristic account of a cranium, however, it is nessary to take 

 into consideration not only the configuration of the bony 

 skull of the adult specimens, but also the chondrocranium 

 and the modes in which the skeleton-elements develop. As 

 to the chondrocranium of the Plectognaths nothing is known 

 except what I have stated above as regards certain stages 

 of Mola and Spheroides. To judge from this the trabecle 

 does not seem to reach the cranial base. The number of 

 semicircular septs is only two. To compare the chondro- 

 cranium of these two Plectognaths with those of other tele- 

 osteans, however, is of no use until we know more about 

 these. Very few teleosteans have been studied from this 

 point of view. 



C. Paired Fins. 



As regards the skeleton of the paired fins I have no- 

 thing to add to what previous writers have stated. The supe- 

 rior supracleiihral is a short simple piece of bone, which, in 



