PHYLOGENY OF THE PELECYPODA. 281 



One of the most important diseovei-ies described in this paper is the existence in 

 young Pelocypods of an ('mhryomc shell, or prodissoconch comparaMe to the emhrijonic 

 shell or protoconeh of cephalous molluscs. The sliell of Pelecypods develops from a 

 131-econchyUau gland, as does the shell of cephalous molluscs; therefore, the two valves 

 of a Pelecypod are together homologous with the single valve of univalves as was shown 

 by Brooks (7). The adidt shell of cephalous molluscs has been named the conch, or 

 true shell, and the first formed embryonic shell on account of its differences the pro- 

 toconch; as the adult double shell of Pelecypods is the homologue of the adult single 

 shell of univalves I have named it tlie dissoconch (double shell) ; similarly the early em- 

 bryonic double shell on account of its differences I named the prodissoco7ich (early 

 double shell). A prodissoconch is described in this paper in twenty-four genera of Pe- 

 lecypods,^ figures of most of which are given in the plates. It has been found in many 

 species of some genera. These genera are widely separated so that it may be considered 

 as proven that a prodissoconch is a chai-acteristic feature of developing Pelecypod shells 

 (discussion and descriptions, sections vi to xv inclusive). 



In the discussion of the prodissoconch it will be seen that stress is laid on the direc- 

 tion of the umbos, as to whether they are inclined anteriorly or posteriorly relatively to 

 the anatomy of the animal at the close of the prodissoconch period. I find that they are 

 inclined posteriorly in Avicula, Perna, Ostrea, Pecten, Sp^ndylus, Plicatula and Anomia. 

 On the contrary they are inclined anteriorly in the other genera described except the 

 Unionidte. The value of this feature may be overrated, but I think it is of importance 

 in considering the connection of genera and the derivation of the prodissoconch (sec- 

 tion xv). 



In considering what value is to be attached to inclination of the umbos, attention 

 should be called to two singular, ancient palaeizoic genera, Autipleura and Dualina, 

 described by M. Barrande. In single species of these genera series of specimens may 

 be picked out in which the umbos are inclined in one direction, and other series may 

 be picked out in which the umbos are inclined in ths opposite direction; the two series 

 being apparently identical in every other character. This is a puzzling case, seemingly 

 opposed to all natural laws. M. Barrande figures large numbers of specimens ilhistrat- 

 ing the two series, and I have studied th^m in the Schary collection of fossils from 

 Bohemia, now in the M iseum of Comparative Zoology). Ordinarily, however, the in- 

 clination of the umbos is a constant character and therefore I think it is not a mistake to 

 consider it a feature of serial value as I have done in the text. 



The prodissoconch of Ostrea, Pei-na, Pecten, Anomia and allies, is composed of 

 homogeneous sub-nacreous lime showing no prismatic clelular layer, fig. 27, section vi, 

 the first stages of the dissoconch growth on the other hand in these genera possesses a 

 prismatic cellular structure making a m irked contract in histological structure with the 



'Previous to the publication of my preliminary papsr, C23cliug dissoconch sh?U in Pelecypods. In this paper a 



an early embryonic shell and its relations to ths succjcd- prodissoconch is described in the foil owing .a:enera: Ostrea, 



ing shell had been described at some length by Ryder in several species; Gryphiea ; Exogyra; Perna; Avicula; Pec 



the oyster, briefly by Morse in Anomia, and briefly by ten, several species ; Spondylus ; Plicatula; Lima; Anomia 



Lacaze-Duthiers in Mytilus. With tliese exceptions I be- two species; Mytilus; Modiola, Crenella: Argina; Unio ; 



lieve my descriptions are the flrst concerning the early Margaritana; Anodon; Spha-riuni; Pc-tricola; Scrobicn. 



embryonic, prodissoconch shell and its relation to the sue- laria; Venus; Tottenia; Mya and Saxicava. 



