PHYLOGENY OF THE PELECYPODA. 343 



The siicceofling dissoconeli growth in Pecten is clearly and sharply separated off by 

 its c'liang-ed form and histological strnctnrc. A^hereas tlic prodissoeonch shell is homo- 

 geneous and laminar in strnctnre, the succeeding dissoconeli of Pecten irradians, in its 

 early stages, PI. xxviir, 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- 

 omia, as shown in later studies. The early stages of the dissoconeli growth of Chlamys 

 (Pecfeii) islandicus, Chenin., Pecten dislocattis, Say, P. viagellauicus, Gmelin and P. 

 monotimeris, Con., also show mai-ked prismatic cellular structure in the right valve. The 

 prismatic stn cture of these sevei-al species does not occur in patches here and there, 

 l)ut extends over the whole eai'ly dissoconeli shell in a thin but clearly marked layer. 

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

 of prismatic structure, 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 Pectiuidse; but believes that it is not to be considered as constant or pe- 

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

 since found prismatic cellular structure, thin, but beautifully preserved in Pecten nohilis, 

 which he figures. This is the only case I find mentioned in which prismatic structure 

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

 has observed prismatic cellular structure in many species of Ainusium, and in the allied 

 genera Pseudamusium and Propreamusium. In these cases the jjrismatic structure, 

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

 tions of young 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 structure 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 young. The 

 Aviculidse, in which prismatic structure is highly characteristic, are in the line of ances- 

 try of the Pectinidse. 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. 



Although prismatic structure is characteristic of the right valve in young Pecten irra- 

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

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

 structure. The degree of pitting varies in ditterent individuals, but in none Avas it as 

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



In Pecten the shell of the fii'st true dissoconch or nepionic period, PI. xxviii, figs. 9- 

 10, is markedly different from succeeding stages, as well as from the eaily completed 

 prodissoeonch 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 

 boi'der 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 although 

 these originate early and are characteristic of the adult. The features of the ventral 

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



