THYLOGENY OF THE PELECYPODA. 293 



but, instead, a well-defined cicatrix marks the position wliere the protoconcli has become 

 separated off. The cicatrix consists of granular lime and fills as a plug the space which 

 otlierwise would be a foraminal opening in the shell. In form the cicati'ix simulates the 

 ontline of the base of the separated protoconch as will be evident by comparing the scar 

 of fig. 24 with the protoconch of Crepidula in PI. xxrii, fig. 23. The cicatrix of Acmaea 

 is made up of two areas, one circular and surrounded by a ring of similar nature, the 

 other roughly triangular ovate. I find that a similar outline is formed by laying a small, 

 slightly muddy Gasteropod on a clean glass slide. The closely related genus Patella 

 fi-o'in Patten's studies is known to have a typical spiral protoconch. Thei-efore, there can 

 be no question as to the meaning of the cicatrix in Acmaea. Professor Hyatt assumed, 

 as noted above, that the existence of a cicatrix in Nautilus pi-edicated the presence of a 

 protoconch in the young and it gives me great pleasure to offerthis corroborative evidence 

 of the truth of his assumption from another class of molluscs. 



In Scaphopods the embryonic shell has been named a pericouch l)y Professor Hyatt 

 (33) on account of its peculiar saddle-shape form. See Dentaliura, PI. xxiii, figs. 13-U. 

 In Pelecypods the bivalvular embryonic shell or prodissoaonch is figured in many genera 

 in this paper. Examples: see almost every plate. 



JSrejnomc period. The period of the first formation of the true shell which succeeds 

 the embryonic shell and is normally retained throughout the rest of life. The period is 

 commonly characterized by marked stages. Examples: figures of developing Ostrea, 

 Pis. XXIV and xxv, figures of developing Perna, Pecten, Anomia, Arginaand other young 

 Pelecypods figured on Pis. xxvii to xxx, inclusive. Also Professor Hyatt's (27) PI. iv, 

 figs. 7-9, of a yonug miutilus honinckii. These last figures have a smooth, nepionic 

 period, succeeded by a fluted, neologic stage. The nepionic stage is shown in Crepidula 

 fornicata and in Acmaia testudinalis var. cdveus, PI. xxiir, figs. 21, 23, 24. It may be 

 observed in Sycoto[)us, Fulgur and Bnccinum when removed from their g^q cases, in 

 which condition the shell is very finely preserved. 



Nealoqlc jjerlod. The period succeeding the nepionic and preceding that period which 

 may properiy be considered as the adult. It is frequently characterized by mai-ked 

 stages, being a period in which the gi-owing animal often differs widely from tlie adult, 

 and as it is of considerable size, the diff"erences and stage are easily recognized. Ex- 

 amples: mnnites cortesi, PI. xxvi, figs. 3-4, has a peeteniform nealogic stage sharply 

 marked oft^ from the ostreafoi-m adult condition. Slava fibrosa from the paleozoic PI. 

 xxui, fig. 12, is one of the most striking cases of definition of the nealogic period that I 

 have ever seen. The nealogic stage as shown in the figure is concentrically plicated and 

 is highly arcuate, whereas the succeeding ephebolic period is radially plicated, spread 

 ont in a fan-like tashion and is much less arcuate. Many species and individuals of this 

 remarkable genus are figured by M. Barrande. The period is strongly marked in the well- 

 known ClavagellaandAspergillum and the last mentioned genus has a definitely marked 

 nepionic stage as well. The young shell of these two genera has up to the close of the 

 nealogic stage two equal valves. In the adult the bivalvular condition is lost; the two 

 valves having united so as to form a single tubular shell, which by itself would not be 

 recognizable as a Pelecypod. In the Gasteropoda, Vermetus, fig. 9, and Magillus, the 

 nealogic period is close coiled, whereas the adult is loosely coiled and vermiform having 



MEMOlltS BOSTON SOC. NAT. HIST., VOL. IV. 



