PHYLOGENY OF THE PELECYPODA. 327 



testiiie and mantle border are also closely alike in Pei-na and Ostrea. In Ostrea the foot 

 is wanting, but probable remnants of it are preserved as the pedal muscle mentioned by 

 Dall, 2^<l-, ^?*1- XXV. fig. 12. The prodissoconch of Ostrea is strikingly like that of Perna. 

 In both genera it is an oval, nearly or quite equivalvular shell with umbos directed pos- 

 teriorly. In both it is composed of homogeneoiis^lime, and shows fine concentric lines 

 of growth. The succeeding dissoconch in both Ostrea and Perna is strikingly different 

 from the prodissoconch. In Perna it is composed of an external laminai- prismatic cell- 

 ular layer with internal nacreous layers. In the oyster the right valve has an external 

 thin laminar prismatic layer, the bulk of the valve being composed of sub-nacreous tis- 

 sue. The left valve, on the other hand, in the young, is composed wholly of sub-nacre- 

 ous lime, not having the typical prismatic layer (vide p. 314). This absence or extreme 

 reduction of the pi-ismatic \ajev in the left valve of the oyster may l)e dne to the mod- 

 ifying influence of fixation or to other causes, for in young Pectens a prismatic layer 

 is found in the right valve although wanting in the left. 



Having seen some important likenesses of the two genera, Perna and Ostrea, let 

 us examine- the dift'ei'ences and see if they can be accounted for. Ostrea is attached; 

 this habit is acquired by many Pelec3q)ods in widely separate groups. The loss of the 

 foot and accompanying byssal notch in Ostrea is easily explained as the result of fixa- 

 tion, for such a reduction and disappearance are desci'ibed in attached Hinnites, Pl.xxvi, 

 figs. 3-4, Spondylus, PI. xxvn, fig. 4, and Pernostrea. The shell of Ostrea is inequivalvu- 

 lar, of rough, irregular gi'owth, especially marked in the lower valve, which is deeply eon- 

 cave; the sub-nacreous tissue is characterized by a shallow camerated structure. This 

 form and the cameration we have shown are the result of the conditions of fixation acting 

 on attached Pelecypod shells. Ostrea has but one cartilage pit in the middle of the line 

 of ligamental tissue connecting the valves, whereas Perna has many pits situated on 

 such a line of tissue, PI. xxvi, figs. lG-18; but this character is variable in Perna and 

 in the young only a single pit exists, figs. 30, 31, p. 329. The nepionic period, as shown 

 in many genera in this paper, is of value in tracing relationships which are with difficulty 

 or not at all traceable in the adult. Therefore we naturally turn to the nepionic period 

 of Ostrea. I have not been able to trace connections between the nepionic stage of Os- 

 trea and similar periods in Perna, Ijut it is to be remembered that Ostrea, even at this 

 eai'ly period, is already attached and highly modified, so that close comparisons are not 

 to be expected. All diflerences seem referable to details, which are explicable on estab- 

 lished bases of argument. We feel justified, therefore, in supposing that Ostrea is a 

 close ally of Perna, the differences between the two genera being largely explained by 

 the effects of the changed conditions of environment acting on the genus Ostrea. Os- 

 trea probably descended directly from Perna or from a close common ancestor of the 

 two genera, and by soldering its shell to a foreign body at the close of the prodisso- 

 conch period has by this means eradicated from the shell, features which might otherwise 

 I'cnder the young referable to Perna in form. 



YIII. Peutsta, Avxcula and near allies. 

 Seeking on masses of Perna ephippinm, L., preserved in alcohol, a immber of young' 

 specimens were secured, which had the embryonic shell well preserved. The young were 



