20 NILS HJ. ODHNER 



In this I think we have to search the homologon of the posterior 

 lobe of Goniaeolis. The anterior one certainly represents the 

 rest of the anterior corner of the notum. 



The reference of Goniaeolis to Pleurohranchaea gets a 

 support in the configuration of the head. Here we find in Goniae- 

 olis, as shown in my figure of 1907 (fig. 2 in this memoir), on 

 each side a tapering furrow-shaped fold emerging from the frontal 

 part of the head, and projecting backward, but this fold is coalesced 

 with the foot along its under side and with its tip, the margins, (upper 

 and lower), however, rising freely. In this side lobe of the head, we 

 have undoubtedly to recognize the homologon of the frontal 

 velum of Pleurohranchaea, which has been fixed to the foot 

 and, later on, has developed a pair of oral tentacles.^ 



As a consequence of the above arguments, we state that 

 Goniaeolis shows in some respects, a striking resemblance to 

 Pleurohranchaea or to similar types. There are, on the other 

 hand, also great differences; thus for inst. the radula of Goniae- 

 olis has a strong rhachian tooth and one lateral one on each 

 side, while, in Pleurohranchaea, there are many laterals and 

 usually no median tooth. Hereto, it is to be remarked that the 

 laterals of the triseriate radula certainly have arisen through a 

 concrescence of those of the multiseriate radula; the denticles of 

 the former are a reminiscence of this development, which can 

 in many cases, be comparatively stated in transitional forms. 

 Except the peculiar structure of the jaws in Pleurobranchidae, 

 certainly an acquired character of this group, a further differ- 

 ence between the two forms is present in the rhinophores, which 

 are columniform in Goniaeolis, while they in Pleurohranchaea 

 still keep a primitive state as enrolled lobes. For these reasons, 

 any close affinity cannot be maintained, but undoubtedly the 

 ancestors of Goniaeolis are to be sought for in Pleurohranchaea- 

 like Opisthobranchs. » 



^ Similar conditions are present also in Pleur ophyUidia. 



