LIMACINID^. 17 



Family LIMACINID^. 



The anatomy of the soft parts of Pteeopoda has led Dr. Paul 

 Pelseueer to the conclusion ^ that pteropods are not entitled to 

 be regarded as a class of mollusca of the same value as the 

 Cephalopoda, Gasteeopoba, Scaphopoda, and LAMELLiBEAifCHiATA. 

 Pteropods, he believes, are not primitive mollusca, but are a derived 

 and recent group, and he completely abstracts certain Primary 

 fossils usually referred to the Pteeopoda from that group, as 

 represented by living forms. Boas had arrived- at a similar con- 

 clusion, though not on such abundant material. Dr. Pelseneer 

 discusses the matter at some length {op. cit. p. 90), and expresses 

 the opinion that real pteropods had no existence as such until the 

 commencement of the Tertiary epoch. He bases his arguments 

 on comparative anatomy, embryology, and palaeontology, in that 

 section dealing with the origin and phylogeny of the whole group. 



He finds that the pteropods are Gasteeopoda in which the adap- 

 tation to pelagic life has so modified their external characters as 

 to give them an apparent symmetry ; that they do not constitute 

 a distinct subclass, nor even an order. They belong to the 

 Tectibranchiate Opisthobranchs, and they differ less from Tecti- 

 branchs than these differ from the other Opisthobranchs. And, 

 he is of opinion that the Thecosomata to which the Australian 

 fossils presently to be described may be referred, have descended 

 from the Bdlloidea ( = Cephalaspidea). 



The general classification of the Pteeopoda amongst Opistho- 

 branchiate mollusca is here adopted, but without prejudice as to 

 the disposal of pre-Tertiary fossils usually relegated to the group, 

 the consideration of which does not fall within the scope of the 

 present work. 



It is not a little remarkable as a phase in the evolution of 

 the Pteeopoda, that immediately the Thecosomata as a group 

 was well established, members of it became extremely numerous 

 in the Lower Tertiaries of Australia. But the assemblage of 

 genera is not such as might have been expected in rocks 

 as old as the Eocene, and it is highly probable that, regarded 



' "Challenger" Reports — Zoology, vol. xxiii.: Pteropoda, part 3: Anatomj-, 

 p. 96. 



^ Spolia Atlantica, pp. 94, 95. 



