PHYLOGENY OF THE PELECYPODA. 349 



catalogue, is a young individual 3 mm. in height. This specimen is figured in PI. xxviii, 

 fig. 14, as viewed from the left side and PI. xxvn, fig. 8, is the umbonal area more mag- 

 nified, viewed fi-om the right side. The prodissoconch is sharply defined and the um- 

 bos are directed slightly posteriorlj^. It bears concentric lines of growth, no indication 

 of a byssal sinus and is homogeneous in structure. The succeeding dissoconch is yel- 

 lowish-brown and in its early stages has a well-defined, prismatic layer in the right 

 valve. In the oldest period of this young specimen no prismatic layer exists, the struct- 

 tn-e being lost thus early in shell growth. The left valve has a punctate sti-ucture as 

 mentioned in some other species. The early nepionic stage as discerned by following the 

 lines of growth is seen" to be similar in form to the nepionic stage of Pecten irradimis, 

 PI. XXVIII, fig. 9. The left valve is marked by a deep byssal notch, PI. xxvii,fig. 8, the 

 lower border of which bears teeth similar to those of Pecten irroAiians and these teeth 

 exist also in the adult of Chlrmiys islandica. At the close of the smooth, early nepionic 

 stage of C. islandica, we find radiating plications similar to those of the adult originat- 

 ing at a definitely marked area, PI. xxviii, fig. 14. New plications develop in succeed- 

 ing growth between those first formed, instead of the first formed being the only ones 

 built as in Pecten iri'adAans. The hinge line is very long proportionately to the width 

 of the shell as compared with the adult condition; the same feature being characteristic 

 of the young of all the species described. The ears in this early stage are nearly equal, 

 whereas in the adult the ventral (anterior) ear is characteristically much larger than the 

 oi^posite or dorsal ear. 



Pecten dislocatus, Say. A number of young specimens of this species are in the collec- 

 tions of the Smithsonian Institution, under the catalogue number G2278. They were 

 dredged off the coast of the Carolinas at several stations. The specimens vary from 3 to 

 13 mm. in height. All the youngest show a prodissoconch like that of Pecten irra- 

 dians, PI. xxvin, figs. 9-10. The prodissoconch bears fine concentric lines of growth, 

 has no indication of a byssal notch and is not prismatic ; also the umbos are directed pos- 

 tei-iorly as in all this group. The right valve has in eai-ly nepionic stages a well-defined 

 prismatic layer, fig. 40, p. 348, the prisms being of about the same size as those in P. 

 irradians and Chlamys islandica, but smaller than those in Pecten magellanicus. Con- 

 versely, the left valve has no prismatic layer; but it has a very charactei-istic pitted struct- 

 ure in its early stages. The pits are not deep, but numerous and very closely related 

 to one another. They disappear with the close of the smooth nepionic stage. A similar 

 structure is described in some other species; but in none, unless P. testce, is it as notice- 

 able and clearly marked as in this species. 



The early nepionic stage of Pecten dislocatus, as ascertained by following the lines of 

 growth, which are particularly well defined in this species, is seen to be of the same form 

 as that of Pecten irradians, PI. xxviii, figs. 9-10. No ears exist at this period and the 

 hinge line in length equals the entire width of the shell. A deep byssal notch exists in 

 the right valve originating with the close of the prodissoconch stage. In later growth 

 plications in the shell originate suddenly as in Pecten irradians, PI. xxviii, fig. 12, caus- 

 ing by their introduction a marked stage as in that species. Ears also develop with suc- 

 ceeding growth and the hinge line gradually loses its great length proportionately to the 



MEMOIItS BOSTON SOC . NAT. HIST., VOL. IV, 47 



