374 ROBERT TRACY JACKSON ON THE 



the siphon exactly correspoBtls to what is found in the adult of Coibnla (see Fischer, fig. 

 855). Corbiila presents in the adult many features found in the early development of 

 Mya and as it is tlie most ancient member of the Myidse, being known from the Trias 

 up, these facts are features of genetic importance in Mya. The great development of 

 the siphon in Mya arenaria is most distinctly a correlation of structui-e induced by its 

 habit of deep burial, and in the development of the individual we find that with increas- 

 ing age as the individual burrows deeper the siphon elongates. 



Professor Ryder (63), in a paper recently published, announces the discovery of a bys- 

 sated condition in young Mya arenaria. Specimens studied at Buzzards Bay did not show 

 this feature, although kept alive for several days; the clams fi-om Chelsea Beach, however, 

 did show it. I had them in glass dishes and many became attached to the sides and bottom. 

 The byssus is produced from the ventral border of the foot at about the position of its ]wint 

 of extrusion from the shell in PI. xxx, fig. 13. The animal attached itself as observed 

 by a few (1-4) colorless threads of great delicacy. The threads were not expanded in 

 disc-like distal termini as in the byssus of Pecten and Mytilus, but were glued to the 

 glass for a considerable jjart of their length. Sharp angles in the line of the attached 

 portion often existed, and separate threads often coalesced proximally. Byssal fixation 

 occurred in specimens varying from 2.7 mm. to 9 mm. in length, and it was evidently a 

 strong habit, for when detached, in a few minutes the fixation was again effected. A 

 byssal cleft is characteristic of Corbula and Sphenia, near allies of Mya, according to 

 authors, and Forbes and Hanley state that a byssal cleft exists in the foot of Mya trun- 

 cata. In a very young specimen of Mya arenaria studied alive, the gills were filament- 

 ous, ciliated and expanded at their tips as shoAvn in the gills of young Ostrea, PI. xxiv, 

 fig. 3. I did not determine certainly whether cross-bars united the tips or not. 



A young specimen of Mya arenaria, PI. xxx, fig. 12, shows the prodissocouch, p, 

 clearly. The prodissocouch is rounded in outline, with umbos directed anterioi-ly. It 

 is glistening and translucent, covered l^y a delicate epidermis and marked hy fine con- 

 centric lines of growth. The prodissocouch may be retained in well i)reserved speci- 

 mens, much older than fig. 12, even in those which are several millimetres in length; but 

 commonly it is lost early from erosion. The prodissocouch is not so sharply mai-kod off 

 from the succeeding growth in Mya as in many genera, because the dissoconch growth 

 has uo striking featui-es which originate with its introduction. It is, however, perfectly 

 clear when preserved. 



Professor Ryder (63) in his paper on 3Iya arenai'ia, published January, 1889, says 

 that he did not find an embryonic shell in young clams such as is characteristic of young 

 oysters. Since that time I have sent him specimens showing the embryonic or larval 

 shell (prodissocouch) of Mya, and he acknowledges their existence in a letter, saying: 

 " The specimens which you have been good enough to send show the larval shjll very 

 distinctly indeed, much more plainly than my own materials, in fact." 



The dissoconch growth of Mya arenaria is covered with a delicate epidermis and is 

 much whiter than the ])rodissoconch. I did not find any trace of prismatic cellular tis- 

 sue in the young shell of Mya. The early dissoconch growth, PI. xxx, fig. 12, is rounded 

 in outline anteriorly, but truncated posteriorly and marked by fine concentric lines of 

 growth. An older specimen of a clam is seen in PI. xxx, fig. 13. The prodissocouch 



