N. ROSÉN, STUDIES ON THE PLECTOGXATHS. 11 



True ribs are absent, while intermuscular bones (»Fleisch- 

 gräten») äre present in the Triodontids, Triacanthids and 

 some Balistids. 



B. The skull. 



Molidae. 



The literature contains several descriptions of the skull 

 of Mola, e. g. by Cleland, Goodsir, Wellenbergh, Lutken 

 and Steenstrup, Supino. Supino has given the latest account 

 (1905), which I can verify in all respects. He has examined 

 ver}^ carefiilly adult specimens. I refer to his excellent 

 figures. 



The following skeleton- elements can be distinguished: supra- 

 occipitale, occipitale laterale, basioccipitale, epioticum, parietale, 

 jrontale, supraethmoideum, ethmoideum laterale, squamosum, 

 sphenoticum, prooticum, alisphenoideum, basisphenoideuvi, para- 

 sphenoideum, vomer, praemaxillare, maxillare, palatinum, ecto- 

 pterygoideiirn, entopterijgoideum, metapterygoideum, quadratum, 

 dentale, articulare, angulare, operctdum, praeopercidum, inter- 

 operculum, subopercidum, liyomandibidare and symplecticum. 

 Five branchial arches are present. Basioccipitale is large, 

 separated by cartilage from occ. lat and prooticum. The 

 lateral occipitals are separated by cartilage from each other 

 as well as from basioccipitale, parietale, epioticum and sup- 

 raoccipitale. As Supino's figures show, the cranium is not 

 perfectly ossified. Some portions of the chondrocranium per- 

 sist forming as has been mentioned, small areas between the 

 bones just mentioned. Praemaxillare and dentale are unpaired. 

 I cannot state anything as to their development, because 

 they are unpaired already in the youngest specimen (mea- 

 suring 1,5 mm. in length) I have had at my disposal. The 

 maxillaries are, in adult specimens, firmly united with the 

 praemaxillary, so that it is impossible to notice any suture. 

 In young specimens, 1,5 mm. in length, however, these bones 

 are not fused. Palatinum is immovably attached to para- 

 sphenoideum, but articulates with the maxillary. Parasphe- 

 noideum is large. Interoperculnm forms a bony portion of a 

 ligament between the lower jaw and the subopercle. Hyo- 



