rilYLOGENY OF THE PELECYTODA. 349 



calalo<>-ne, is a yoiins; iiulividiinl o mm. in lioight. Tliis .spociimMi is llo-urod in PI. xxviii, 

 iii;". 14, asviowc'd from tlu' left side and 1*1. xxvii, tig. S, is tlie uml)onai aiva nioro ma<^- 

 niiie'd, viewed from the riglit side. The prodissoconch is sharply dellned and the nm- 

 bos are directed slightly posteriorI3^ It bears concentric lines of growth, no indication 

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

 in-e being lost thus early in shell growth. The left valve has a punctate structure 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 Peclen irradlaiis, 

 PI. XXVIII, fig. 0. The left valve is marked by a deej) byssal notch, PI. xxvii,fig. 8, the 

 lower border of which bears teeth similai- to those of Pecten irradiaiis and these teeth 

 exist also in the adult of Chlamys islamUca. At the close of the smooth, early nepionic 

 stage of C. islandica, we fiud 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 irradians. 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 si)ecies described. The ears in this early stage are neai-ly equal, 

 whereas in the adult the venti'al (anterior) ear is characteristically much larger than the 

 op{)t)site 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 oif the coast of the Carolinas at several stations. The specimens vary from 3 to 

 13 nun. in height. All the youngest show a ])rodissoconch like that of Pecten irra- 

 dians, PI. xxviir, 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 early nepionic stages a well-defined 

 ])rismatie layer, fig. 40, p. 348, the prisms being of al)out the same size as those in P. 

 irradi((ns and Chkonj/s islandica, but smaller than those in Pecten magellanicns. Con- 

 versely, the left valve has no prismatic layer; but it has a very characteristic 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 s|)ecics; but in none, unless P. tester, 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 particulai-ly 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 eqiials 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 develoj) with suc- 

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



MEMOIRS llOSTON SOC. NAT. HIST., VOL. IV. 47 



