PHYLOGENY OF THE PELECYPODA. 343 



The succoeding- dissoconch growth in Pecten is clearly and sharply separated oif by 

 its ehang-ed Coi-in and histological structure. ^7hereas the prodissoconch shell is homo- 

 geneons and laminar in stiaicture, the succeeding dissoconch of Pecten irradians, in its 

 early stages, PI. xxv^rir, figs. 9-12, has a thin layer of prismatic cellular tissue in the 

 right valve, the same feature having been observed in Ostrea, fig. 27, p. 314; also in An- 

 oniia, as shown in later studies. The early stages of the dissoconch growth of Chlamys 

 (^Pecten) islandicHS, Chemn., Pecten clislocatus, Say, P. mageUanicus, Gmelin and P. 

 monotimeris, Con., also show marked pi'ismatic cellular structui'c in the right valve. The 

 prismatic stricture of these several species does not occur in patches here and there, 

 but extends over the whole early dissoconch shell in a thin but clearly marked layer. 

 This is a most interesting fact as adult Pecten irradians does not exhibit any evidence 

 of jirismatic strnctui'e, though it is well developed in the young. Professor Carpenter 

 in his first paper on shell structure, in 1844, says that he has seen traces of cellular 

 structure in the Pectinidse,; but believes that it is not to be considered as constant or pe- 

 culiarly characteristic of the group. In his later papei- in 1848, he states that he has 

 since found prismatic cellular structure, thin, but beautifully preserved \n Pecteit nohilis, 

 which he figures. This is the only case I find mentioned in which ])i'ismatic structure 

 is known in Pecten. Dr. Dall (16), however, in recent studies of deep-sea molluscs 

 has observed prismatic cellnlar structure in many species of Amnsium, and in the allied 

 genera Psendamusium and Propreamusium. In these cases the prismatic structin*e, 

 though often limited in extent, was not confined to the right valve, as in my observa- 

 tions of 5'oung Pectens. Dall notes that this structure is especially characteristic of 

 abyssal species of this group, in which also the shell is extremely thin and fragile. My 

 observation of prismatic structni-e in the young of five species studied, proves that this 

 feature which is not a characteristic of the group may yet be found in the yoitng. The 

 Aviculida?, in which prismatic structure is highly charactei'istic, are in the line of ances- 

 try of the Pectinidaj. The occurrence of prismatic structure in young Pectens is there- 

 fore a good example of the principle of acceleration of development which is that 

 ancestral characters may be found in the young which are wanting in adult individuals. 



Althougli prismatic sti'ucture is characteristic of the right valve in young Pecten irra- 

 dians, the left valve in this and the other species studied has no prismatic layei". In 

 P. irradians, the left valve in its early nepionic stage is marked by a peculiai' pitted 

 structure. The degree of pitting varies in different individuals, but in none was it as 

 marked a feature as in young P. dislocatus, where it is highly accentuated. 



In Pecten the shell of the first true dissoconch or ne|)ionic period, PI. xxviii, figs. 9- 

 10, is mai'kedly different from succeeding stages, as well as from the early completed 

 prodissoconch stage. The hinge is produced as a straight line and the borders of the 

 valves descend in a curve from the extreme limits of the hinge line without possessing 

 the ears which are characteristic of the adult in all species of the genus. The ventral 

 border of the left valve, fig. 9, has no byssal notch but near the hinge area is produced 

 in a lobe-like curve, the lobe projecting beyond the border of the byssal sinus of the right 

 valve as shown in fig. 10. There are no plications of the shell at this stage althougli 

 these originate early and are characteristic of the adult. The features of the venti-al 

 lobe, and the absence of ears and plications, give the shell as shown in the figures cited 



