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or Lower Cambrian of the Atlantic border province of North America. 

 That it is not the most primitive type of gastropod is suggested by the con- 

 sideration that the earliest stage .... of the protoconch is not 

 coiled, but rather cap-shaped like modern Patella. Such primitive types 

 are found in Lower Cambrian species which have variously been referred 

 to Platyceras, Scenella, or Stenotheca, owing to the want of sufficient 

 characteristics to define their exact relations." (22.) 



From the above it appears that the early protoconch stages indicate 

 an ancestor of the simple, smooth shelled, umbilicated type exemplified by 

 StraparolUna, and that this is actually the only type of coiled gastropod 

 characteristic of the basal Cambrian. It is also likely from paleontolog- 

 ical evidence that the very earliest type of gastropod possessed a conical 

 or cornucopia-shaped shell of the Scenella type. 1 Such an ancestry is, 

 according to Grabau, suggested by the cap-shaped earliest stage of the 

 protoconch. 2 



One of the most completely worked-out cases of recapitulation among 

 Gastropoda that has come to my knowledge is that of the races of Athleta 

 petrosa Con. and its allies. The phylogeny of this group of gastropods 

 has been very fully studied by Burnett Smith (54), from whose paper 

 the following account is drawn. 



1 Sardeson (50) suggests that the gastropod ancestor was an "asymmetrical 

 long conical shell" of the pteropod type. He may be right, but even so, I do not 

 see that his conclusion would in the least invalidate the conclusions of Grabau in 

 regard to the phylogenetic significance of the protoconch, although Sardeson seems 

 to think so. Grabau says very plainly that the coiled shell is probably not the 

 most primitive type of shell, and he points out the fast (quoted above) that the 

 initial portion of the protoconch is cap-shaped and may indicate some such remote 

 ancestor as the Cambrian forms referred to the genera Platyceras, Stenotheca, and 

 Scenella. Whether this patelliform ancestor was in turn derived from a long 

 conical shell, or whether on the other hand the coiled type of shell was derived 

 directly from the "long conical" shell without the mediation of a patelliform 

 ancestor, does not materially affect the conclusions that at a very remote time 

 a coiled gastropod radicle was established from which practically all modern 

 gastropods were derived. To my mind the conclusion that the ultimate ancestor 

 of the Gastropoda was a "long conical" shell is by no means established. 



2 Burnett Smith (55) concludes from a study of the Tertiary species of the 

 genus Athleta that "we can say for this restricted normal group at least that the 

 apes is not only a variable feature, but the most variable feature which the shells 

 furnish." In a footnote he says "The author is thoroughly convinced that the 

 features of the apex must be used in classification with great caution." The varia- 

 tions which he cites in this and other papers (54, 55, 56) seem to be chiefly in 

 the size of the protoconch, and the degree to which acceleration has caused conchal 

 characters to appear in the later protoconchal stages. His caution, however, in 

 regard to the classificatory value of the protoconch, should put students of the 

 gastropods on their guard against a too free use of this portion of the shell in the 

 establishing of genera. 



[21—23003] 



