PHYLOGENY OF THE PELECYPODA. 3G5 



of growth, and is very yellow compared with the succeeding dissoconch growth which is 

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

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

 Aix-as. Its first stages of growth 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 

 portion a distinct depression exists on the border of the valves and also in the preceding 

 lines of gi-owth. The early dissoconch before the introduction of plications ditfers ninch 

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

 Arcida;. 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 young Argina are oblique like 

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

 but at an earlier stage of growth 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 depths has all the teeth oblique like those on the distal 

 limits of the hinge line in young 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 Ai'ca, the type- 

 genus of the family. Transitional series of specimens of A.rgina 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 

 there are four distinct stages which represent the ancestral forms from which the genus 

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

 nnknown; (2) the Macrodon stage, comparable to pahvozoic forms of the family; (3) the 

 Ai'ca stage, comparable to the type genus of the family and, finally (-t), the Argiiia 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 pexata has been found in 

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

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

 alleloplpedam tortuosmn, Lam., is also similar to the same stage of Argina, the tortuous 

 character of the adult shell not having been acquired at this early period of growth. 



It is an important fact that members of the Arcidie 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 the young 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 Aviculida} and their allies. 



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

 by a peciiliar 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- 



MEMOIKS UOSTON SOO. NAT. HIST., VOL IV. 4'J 



