PHYLOGENY OF THE PELECYPOBA. 



375 



of this later stage, on account of the arcuateness of the umbos, is not seen so clearly in 

 side view as in the younger stage. The shell ditTers much from the adult in form, being 

 less rounded anteriorly and less pointed posteriorly, also the valves gape less proportion- 

 ately than in the adult. An angular ridge runs from the umbos posteriorly to the ven- 

 tral border of the valves, emphasizing a liattened area above it. The features described 

 are found in older young specimens, but gradually fade out and in successive growth 

 give place to the characteristics of the adult. 



The young shell of Mya arenaria bears a marked resemblance to some species of 

 Corbula (leaving out of consideration the inequality of valves in many species of that 

 genus) ; especially does it resemble C. contracta. Say, of our coast. The anatomy of 

 young Mya also bears a close resemblance to the adult of Corbula (see Fischer's fig. 

 855) in possessing a similar siphon, PI. xxx, fig. 13, papillose tentacles on the mantle 

 border at the point of extrusion of the foot and a byssated foot. As Corbula is the old- 

 est known genus of the MyidsB, being found from the Triassic upwards, and as it pre- 

 sents similar features to those found in the young of Mya, it is probably in the line of 

 ancestry of the Myas and is a near representative of the form from which they have 

 descended.^ 



XY. On the Genetic Relations of the Prodissoconch.^ 

 In the development of Pelecypoda at an early embryonic stage, which I have described 

 as the phylembryo stage, the two valves of the shell are sub-circular or saucer-shaped; 

 but flattened into a straight line at the hinge area where they are joined by a ligament. 

 This stage is characteristic of many developing Pelecypods (PI. xxiii, figs. 7-11; PI. 

 xxw, figs. 13-16 and fig. 25, p. 300)' and apparently represents a primitive ancestral 

 condition common to the whole class. Embryology shows that the Pelecypod shell orig- 

 inates from a single shell gland and that the two valves doubtless arose from the split- 

 ting on the median line of a primitive univalvular ancestral form. If that ancient 

 univalvular radical had a saddle-shaped shell as in young Dentalium, PI. xxrii, fig. 13, 

 or a cup-shaped shell as exists in very early stages of cephalous mollusca,* the first re- 

 sult of the introduction of a median hinge would be to straighten the shell on the hinge 

 line. This is a simple problem in mechanics ; for, if one tries to break by flexion a piece 

 of metal which is saddle-shaped or cup-shaped, the metal will tend to form a straight 



' Sincf this paper was in press, a prodi-^soconcli lias been 

 cUscovori'd in a number of genera not considered in the 

 forejjoing pages. An ovally rounded prodissoconch has 

 been found in Nucvla proxima and in Tellina. In both 

 genera tlie umbos of the prodissoconch are directed pos- 

 teriorly as in the adults A prodissoconch has also been 

 discovered la Chione, Cardita, Cardium, Mactra and a 

 number of undetermined genera belonging to other families 

 of Pelecypods. In these latter the umbos of the prodlsso- 

 couchs are directed anteriorly, and the succeeding disso- 

 conch Is destitute of prismatic cellular tissue. 



'This section and the next one, xvi, have been con- 

 siderably altered and added to since this paper was ottered 

 asathesisat Harvard University on May 1. ISti'J. Thead- 

 dltions are not indicated excepting by this note. 



^It is also described in Teredo by Hatschek. Excep. 

 tions to the rule occur as in I'isidiura, where according 

 to Lankester (-13} the early embryonic valves are triangu- 

 lar. I think such cases may be considered exceptions in 

 which the straight hinge period is skipped by acceleration 

 of development. 



■■The shell of Gasteropods, as I have observed in stud- 

 ies of Sycotopus, is cup-shaped before it acquires the 

 spiral form of the completed protoconch, and a similar 

 condition is shown In Lankester's (4.'?) figures of Aplysia. 

 The shell of Pteropods and Cephalopods is also at first 

 cup-shaped as Is obvious in studying the changes that 

 must take place in the building up of the completed pro- 

 toconch, PI. xxui, figs. 15-18. 



