PIIYLOGENY OF THE PELECYPODA. 327 



testine ;iiul mantle border are also closely alike in Porna and Ostrea. In Osti'ca the foot 

 is wantinji;', bnt pi'obable remnants of it are preserved as the pedal muscle mentioned by 

 T) aW, jxl, PI. XXV, fig. 12. The prodissoconch of Ostrea is stiikingly like that of Perna. 

 In both genei-a it is an oval, neai'Iy or quite equivalvular shell with umbos directed pos- 

 teriorly. In both it is composed of homogeneous 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 laminar prismatic cell- 

 ular layer with internal nacreous label's. In the oyster the right valve has an extorual 

 thin laminar prismatic layer, the bulk of the valve being composed of suI)-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 

 rcKluction of the prismatic lajev in the left valve of the oyster may be due to the mod- 

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

 is found ill the rio-ht valve althou<>'h wantiuij: in the left. 



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

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

 this habit is acquired by many Pelecypods 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 described in attached Hinnites, PI. xxvi, 

 figs. 3-4, Spondylus, PI. xxvii, fig. 4, and Pernosti-ea. The shell of Ostrea is inequivalvu- 

 lai', of rough, irregular growth, especiallj' marked in the lower valve, which is deeply con- 

 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 ])it 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. 1(3-18; 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, but it is to be remembered that Ostrea, even at this 

 early pei'iod, is already attached and highly modified, so that close comparisons are not 

 to be expected. All differences seem referable to details, which ai-e explicable on estab- 

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

 close ally of Perna, the diftei-ences 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 tie prodisso- 

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

 render the young referable to Perna in form. 



YIII. PiSllXA, AviCULA AND NEAR ALLIES. 



Seeking on masses of Perna epltippbmi, L., preserved in alcohol, a number of young 

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



