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, Spirula, 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 Vau#- 
Zus 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 Zentaculites as the radicle of the stock 
of cephalopods, although this is no great help, for the 
systematic position of Zentaculites, a supposed pteropod, 
* Mem. Boston Soc. Nat. History, vol. iv, No. 8, 1890. Phy- 
logeny of the Pelecypoda. 
