rilYLOGENY OF THE PELECYPODA. 293 



but, instead, a well-defined cicatrix marks the ])osition Avheiv the protoconch has become 

 separated oft*. Tiie cicatrix consists of granular lime and tills as a plug the space which 

 otherwise would be a foraminal opening in the shell. In form the cicatrix simulates the 

 outline 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 uj) of two areas, one circular and surrounded In' a ring of similar nature, the 

 other )-oug]dy 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-om Patten's studies is known to have a typical spii'al protoconch. Therefore, there can 

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

 as noted above, that the existence of a cicatrix in ^N^autilus predicated the i)resence of a 

 protoconch in the young and it gives me great pleasure to otter this corroboi"ative evidence 

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



In Scaphopods the embryonic shell has been named a perlconch by Professor H3'att 

 (33) on account of its peculiai- saddle-shape foi'm. See Dentalium, PI. xxiir, figs. 13-14. 

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

 in this paper. Examples: see almost every plate. 



Nejyionic pe7'iod. The pei'iod of the first formation of the true shell which succeeds 

 the embryonic shell and is noi-mally 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. xxvn to xxx, inclusive. Also Professor Hyatt's (27) PI. iv, 

 figs. 7-9, of a young JVautilus koninckii. These last figures have a smooth, nepionic 

 ])eriod, succeeded by a fluted, neologic stage. The nepionic stage is shown in Crepidula 

 fornicata and in Acmcea testudinalls var. cdveus, PI. xxirr, figs. 21, 23, 24. It may be 

 observed in Sycotopus, Fulgur and Buccinum when removed from their egg cases, in 

 which condition the shell is very finely preserved. 



JVealof/ic period. The period succeeding the nepionic and preceding that period which 

 may properly be considered as the adult. It is frequently' characterized by marked 

 stages, being a period in which the growing animal often difters widely from the adult, 

 and as it is of consideralile size, the ditfercnces and stage are easily recognized. Ex- 

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

 marked off" from the ostreaform adult condition. Slava fibrosa from the paleozoic Pi. 

 XXIII, 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 concenti'ically plicated and 

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

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

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

 known Clavagella and Aspergillum 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, whicli by itself would not be 

 I'ecognizable as a Pelecypod. In the Gasteropoda, Yermetus, fig. 9, and Magillus, the 

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



MEMOIHS BOSTON SOC. NAT. HIST., VOL. IV. 40 



