PHYLOGENY OF THE PELECYPODA. 363 



era now taken up, excepting the Unionidae, two marked features of difference occtir as 

 compared with those previously studied. The prodissoconcJis are directed more or less an- 

 teriorly, and the succeeding dissoconchs are not characterized hy prismatic cellular 

 structure. 



The Pelecyj)oda is a chxss in which it is very difficult to find binding characters on 

 which to make systematic divisions. The direction of the prodissoconch umbos and the 

 condition of prismatic structure in the succeeding dissoconch, I am aware, is not wholly 

 confined to the Avicnlida3 and their allies, but these features are characteristic of the 

 whole of that group and are not together charaetei'istic of any othei- group so far as 

 known, excepting, possibly, the Unionidje. A thin, but well-defined, prismatic layer ex- 

 ists in Pandora rostrata, Lam.; a prismatic layer also exists in the deei^er-seated portion 

 of the periostracum of Solemya mediterra7iea, Lara., and it has been observed in limited 

 patches in young Ensis vlridis, Say. Carpenter observed the j^rismatic layer in the two 

 former genera and mentions finding traces of it in other genei-a, but he was impressed 

 with the fact that it is a marked feature in the group of the Aviculidae and allies. 



Mytilus edulis, L. This mollusc belongs to one of the oldest families of the Pelecj'^- 

 pods extending back to early paheozoic times. It is therefore interesting to know the 

 conditions existent in the young shell. In PI. xxix, fig. 9, it is shown that a well- 

 defined prodissoconch,^, exists which is yellowish-brown and marked by tine concentric 

 lines of gi'owth. The form of the prodissoconch is evenly rounded and it has well de- 

 veloped umbos which are directed anteriorly. The succeeding dissoconch growth of 

 M3'tilus is sharply marked off from the prodissoconch stage by its changed form and 

 color. In form the dissoconch hasali'eady acquired features characteristic of the group 

 to which it belongs, though not of the genus itself. The hinge line is produced poste- 

 riorly in an arched line and anteriorly the lines of growth are curved inwards toward 

 the umbonal area, but they are not crowded closely as in the adult or any species of the 

 genus. The shell at this age reminds one strongly of forms seen commonly in the 

 Modiomoi'phas, palasozoic kindred of the Mytilidae. In color the dissoconch is distinct 

 from the prodissoconch. Whereas the latter as stated is yellowish-brown, the dissoconch 

 is characterized by the violet color characteristic of the adult. The transition from brown 

 to violet color in Mytilus is as sharply defined as is the change from homogeneous lime 

 in the prodissoconch of Ostrea to the prismatic structure of the succeeding dissoconch, 

 fig. 27, p. 314. Specimens of Mytilus edulis of the light horn colored variety of course 

 do not show this change to violet with the introduction of the dissoconch period. No 

 trace of prismatic cellular tissue has been found in the shell of Mytilus or other member of 

 this family at any period of growth, but a well defined epidermis exists.' Lacaze-Duthiers 

 (41) published an anatomical figure of young Mytilus edulis compai'able to the prodisso- 

 conch stage and he notes the striking difference between the shell of that age and the 

 . adult. Loven also published an anatomical figure of developing Mytilus which compares 

 favorably in its shell form with PI. xxix, fig. 9. 



An older specimen oi Mytilus edulis, PI. xxix, fig. 10, shows no important differences 

 from the younger specimen, fig. 9. The prodissoconch valves on account of the increased 

 arcuateness of the umbos are not seen in side view as well as in the earlier stage, a 



'I'risuiatic strvicturc is (k-.scrilKd in tlic iiiiu r layrrs of Drcissona, but lliis geiuis has iiul been slndiid. 



