ZOOLOGICAL AFFINITIES. 11 



surprise that a theory, not possessing even the appearance of probability, should 

 have been accepted as a fact without any kind of proof. He considered the 

 determination of the systematic position of Gonularia to be a matter of great 

 difficulty, but suggested that it is most probably allied to the Palaeozoic 

 Capulidse. 



In 1881 von Ihering l placed Gonularia with the other so-called Palaeozoic 

 Pteropods in the class with which it was first associated. He considered that 

 these simple chambered shells constitute the earliest and most primitive group of 

 Cephalopods, allied to the Orthoceratidee, and named by him, on account of the 

 thinness of the shell, " Leptoceratiten." From this group, according to von 

 Ihering, arise on the one side the Tetrabranchs as a small offshoot, and on the 

 other the Dibranchs (in which he included the Ammonites) as the main branch. 



Lindstrom, 2 in 1884, criticised these conclusions. He stated that in his opinion 

 the external similarity of some species of Clio (or Gliodora) to a Conularian shell 

 was most striking ; that the internal longitudinal septa found in a few species 

 (e. g. G. asjpersa) were homologous to similar structures found in Clio, Styliola, etc; 

 that the typical ornamentation of a Conularian was approached in some species of 

 Glio ; and finally, that among living Pteropods forms differed sufficiently widely 

 to make any discussion of close correspondence unnecessary. He therefore 

 retained the genus with the Pteropods. 



In 1889 Pelseneer :i endeavoured to prove that the " Pretendus Pteropodes 

 Primaires " had no connection with the modern Pteropods. He based his 

 conclusions on : (1) The form and character of the Conularian shell, and (2) the 

 probable origin of the Pteropods. Although his arguments were answered, each 

 in turn, by C. Wiman 4 in 1894, it appears to me that his deductions as to the 

 affinities of modern Pteropods cannot be refuted. 5 



After prolonged study Pelseneer has come to the conclusion that the Pteropods 

 are not a primitive, but a highly specialised group, derived from two different 

 sources: (1) the Thecosomatous Pteropods (with which Gonularia was associated) 

 from Bulloidea-like Opisthobranchs, and (2) the Gynmosomatous Pteropods from 

 Aplysioidea-like Opisthobranchs. 



The evidence for the highly specialised nature of the Pteropods is seen in their 

 marked asymmetry, and the great concentration of their nervous system ; and in 

 their embryology. As the embryo is even more asymmetrical than the adult, they 

 must be derived from ancestors already highly specialised, and the apparent 

 symmetry is acquired by adaptation to natatory habits. 



1 H. von Ihering, ' Neues Jahrb. fur Min., etc' (1881), vol. i, p. 87. 



2 G. Lindstrom, ' Silur. G-astrop. and Pterop. of Gotland' (1884), pp. 39, 40. 



3 P. Pelseneer, ' Bull. Sue. Beige de Guol., etc.,' vol. iii (1889), Mem., pp. 126—136. 

 » C. Wiman, 'Bull. Geol. Inst. Upsala,' vol. ii (1895), pp. 114—116. 



5 See also Pelseneer, ' Eep. Challenger Expedition,' vol. xxiii. pt. iii (Anatomy), pp. 75 — 96. 



