PHYLOGENY OF THE PELECYPODA. 313 



The prodissoconch of Ostrea virginiana is homogeueoas and laminar in arrangement, 

 "not prismatic" as in tlie immediately succeeding stages of spat growth as described and 

 figured by Ryder (G2). It is composed of lime infiltrating an amorphous matrix of con- 

 chyolin, as shown by treatment with acid. One of the marked characters of the prodis- 

 soconch is the uniformity of shape and size found in different individuals, it not having the 

 ostrean tendency to variability noticeable in later stages. The height of the fully de- 

 veloped prodissoconch is about g^ of an inch. The left attached valve turned over and 

 viewed from the lowei- side is seen to be exactly like the I'ight free valve in appearance. 

 It is not flattened nor are there other indications to mark where it was attached, so that 

 this attachment is evidently of a very superficial and delicate nature. 



A prodissoconch identical, or very closely similar, to that found in Ostrea virgiiiiana has 

 been found in many species of the Ostreadas. Ryder (62) has obsei'ved it in four species: 

 Ostrea equestris, Say, from Florida; O. conchaphila, California; O. cucuUata, Viti Isl- 

 ands, Pacific; and O. peruviana, Peru. I have observed it in undetermined oyster 

 spats, from Cuba, and from the chamber of a Nautilus pompilius, from the Pacific. I 

 also found ostrean prodissoconchs in the following: the spats of tertiary oysters from 

 Carolina; Gryphcea vesiculuris, Lam., Cretaceous, found in both European and American 

 specimens; G. calceola, var. nebrascensis, M. & H., PI. xxiv, figs. 23-24, from the Juras- 

 sic of Wyoming Territory, specimen number, 1881, U. S. National Museum ; Exoyyra 

 conica, Cretaceous, Europe; E. columba, Lam., Cretaceous, Europe; JE. arietina, Creta- 

 ceous, America; and E. costata, Say, Cretaceous, America. Of this last species abun- 

 dant very young specimens, which I collected at Delaware City, Delawai'e, showed the 

 ty])ical early prodissoconch. 



From the number and wide distribution of the species, it cannot be doubted that the 

 prodissoconch found in Ostrea virginian,a is characteristic of the Ostreada3. The teeth 

 noted above in the prodissoconch of O. edulis is the most striking difference that has been 

 found in the prodissoconch of the family. 



There is, in the prodissoconch and spat stages of the oyster, an oi'ganic eonch3'olin at- 

 tachment of the shell itself, thus described by Ryder (62) : "The cementing material 

 seems to be the oi-ganic matrix of the shell which forms a perceptible layer on the out- 

 side of the valves, and which constitutes the epidermis or periostracum of the oyster." 

 In the prodissoconch, as well as spat stage, the cement is so firm that, as Ryder saj's 

 (62), the shell may be broken before it can be removed. To prove that this is an organic 

 cement, if we put a drop of water on a dead, young shell and allow it to remain a short 

 time, it can be readily picked up on the point of a knife. Quite large spat may be re- 

 moved when dead («. e., the organic matter inert) by soaking or boiling. Treatment 

 with acid, on the other hand, does not loosen the hold of the shell. Quantities of young 

 spat die during the summer, yet few dead shells are found on cultch, because soon after 

 death the organic cement decays, is dissolved, and the shell falls off". Professor Huxley 

 notes that the Eui-o])ean oyster separates naturally from unfavorable objects of suppoi-t; 

 and that the separation takes place naturally at an eai'ly stage can be seen mai'kedly in 

 many fossil membei's of the family. Couversel}^, the calcareous ping of Anomia, Avhich is 

 fixed to the object of support by a purely calcareous union, remains indefinitely after the 

 death and separation of the individual to which it belonged. All the evidence is there- 



