PHYLOGENY OF THE PELECYPODA. 365 



of g-fowth, and is very yellow coinpai-cd with the succeeding dissoconch growth which is 

 pure lime white, thus indicating a somewhat more marked conchyolin hasis of the prodis- 

 soconch. The succeeding dissoconch has the typical hinge line characteristic of the 

 Areas. Its first stages of growUi are smooth and marked by nearly concentric lines of 

 growth; but with no plications; well defined plications exist, however, in the later growth. 

 Taking the shell as a whole, the height (measuring from the free border of valves to 

 the hinge line) is much greater posteriorly than anteriorly. In the middle of the ventral 

 ])ortion a distinct depression exists on the border of the valves and also in the preceding 

 lines of growth. The early dissoconch before the introduction of jilications differs mnch 

 in appearance from the adult and bears a strong resemblance to fossil members of the 

 Arcidtc. Especially does it resemble species of Macrodon from the Devonian as figured 

 by Hall. The teeth on the distal limits of the hinge line of yonng Argina are obliqne like 

 those of Macrodon, and between these oblique teeth are the perpendicular Area-like teeth ; 

 but at an earlier stage of gi-owth the perpendicular teeth were apparently wanting, and 

 then the teeth of young Argina would have been closely comparable to those of Macrodon. 

 Area ectomata, Dall, from abyssal dejjths has all the teeth oblique like those on the distal 

 limits of the hinge line in yonng Argina and like those of the early and later species of 

 Macrodon. 



The stage represented by the whole shell of PI. xxx, fig. 2, differs in form from the 

 earlier Macrodon stage and also from the adult, and is comparable to Area, the type- 

 genus of the family. Transitional series of specimens of Argina pexata older than PI. 

 xxx, fig. 2, show in constantly increasing degree the oblong-ovate form and shaggy ep- 

 idermis characteristic of the adult. In the ontogeny of Argina, therefore, we find that 

 thei-e are four distinct stages which i-epresent the ancestral forms fi-om which the genus 

 was evolved: (1) the prodissoconch representing a primitive ancestral radical, at present 

 imknown; (2) the Macrodon stage, comparable to palaeozoic forms of the family; (3) the 

 Area stage, comparable to the type genus of the ftimily and, finally (4), the Argina stage 

 in which the features of the genus and species are introduced and fixed. A prodisso- 

 conch and nepionic stage similar to that described in Argina joexata has been found in 

 a young specimen of Area harhata, L., kindly given me by Professor Verrill, and in 

 some young Areas of an undetermined species fi-om Japan. The nepionic stage of Par- 

 allelopipedam tortuosum, Lam., is also similar to the same stage of Argina, the tortuous 

 character of the adult shell not having been acquh-ed at this early period of growth. 



It is an important fact that members of the Arcida3 have a prodissoconch, because as 

 one of the oldest living groups of Pelecypods it shows the widespread existence in time 

 as well as genera of a distinctly marked early embryonic shell. Species of so many and 

 such widely separated families possessing a prodissoconch in theyoilng is strongest proof 

 that it is characteristic of the whole class of Pelecypods. We also see that the nepionic 

 stages in members of this family can be made use of in tracing genetic relations, as in 

 the Aviculidae and their allies. 



The Unionidae are naturally turned to in studies of this nature as they are characterized 

 by a peculiar early stage called the glochidium. Although the glochidium is known by 

 itself, I do not think the relation of this early shell to the adult has been previously studied. 

 Through the kind intervention of Dr. Charles E. Beecher, I recently received some ma- 



Mi:MOIlt8 bOSTON SOC. NAT. HIST., VOL IV. 49 



