240 



FOOT-NOTES TO EVOLUTION. 



eny, for of the Gastropoda we have not as yet a bioge- 

 netic classification, and the larval stages even of living 

 forms have not been well studied. 



Pelecypoda. — Almost all that has been done in com- 

 paring genera of Pelecypoda with stages of growth is the 

 work of Dr. R. T. Jackson,* who has shown that they 

 all go through a phylembryonic stage, prodissoconch, 

 analogous to the protegulum of Brachiopoda, the proto- 

 conch of Cephalopoda and Gastropoda, and the protaspis 

 of trilobites. The prodissoconch is a straight-hinged, 

 two-muscled, smooth-shelled, bivalve stage, correspond- 

 ing to the nuculoids, the primitive radicle stock of pele- 

 cypods. Even the monomyarian oyster goes through 

 this dimyarian stage. 



Cephalopoda. — The living dibranchiate cephalopods, 

 Octopus, Loligo, Spirilla, Argonauta, and other naked 

 forms, are scarcely capable of preserving their larval 

 stages as fossils ; but what is known of their develop- 

 ment points to a tetrabranch ancestry, as, for example, 

 the rudimentary second pair of gills in some forms. 



The tetrabranchiate cephalopods, of which the Nauti- 

 lus is the only living representative, are entitled to 

 speak with especial authority on evolution, for near the 

 end of a long and varied family history they have gone 

 through all the changes of progression and retrogression 

 of which the group is capable. Also a much more per- 

 fect record has been kept of them in the rocks than of 

 any other class of animals, so that practically all the 

 sorts of tetrabranchs that existed are known. 



Their remote ancestry is unknown, but indications 

 seem to point to Tentaculites as the radicle of the stock 

 of cephalopods, although this is no great help, for the 

 systematic position of Tentaculites, a supposed pteropod, 



* Mem. Boston Soc. Nat. History, vol. iv, No. 8, 1890. Phy- 

 logeny of the Pelecypoda. 



