PHYLOGENY OF THE PELECYPODA. 



361 



not have a prismatic layer at any period in the life of the individual; but is sub-nacreous 

 excepting for a small area around the point of insertion of the muscles, where as in the 

 right valve there is a deposition of porcellaneous tissue. This difference in the structure 

 of the two valves is a remarkable feature and occurs in related genera, for I have shown 

 that a thin prismatic layer exists in the right valve of young Pecten and in the riglit 

 valve of young Ostrea but not in the left valve of either genus in early stages. 



I have not found any published observations of prismatic structure in Anomia glabra, 

 but Carpenter, in his second paper (1848), on shell structure, stated that a thin external 

 prismatic layer exists in Avoniia ephijiplnm; but is early worn off. In this connection, 

 an interesting point is recorded in one of Dr. C. A. "White's papers,^ that among Tertiaiy 

 fossils it is rai-c to find a right valve of Anomia although left valves are abundant. He 

 ascribes the loss of right valves to their fragility owing to their prismatic composition. 

 On the other hand, a number of species of European Tertiary Anemias in the collections 

 of the Museum of Comparative Zoology 

 have both valves equally preserved. A very 

 thin layer of prismatic tissue exists in the 

 right valve of Anomia ejihippmm, as ob- 

 served in specimens Idndl}' sent me by Dr. 

 Dall ; but it does not exist in the left valve of 

 that species, both valves being composed 

 wholly or for the most part of sub-nacreous 

 tissue, with the addition of porcellaneous tis- 

 sue arottnd the mnsetilar insertions as in An- 

 omia glabra. The difierence in the structure 

 of the right valve of Anomia glabra makes 

 a good specific distinction, which separates 

 that species clearly from A. ephijjpiiim, which 

 it approaches very nearly in external form. 



Young specimens of Anomia aculeata, 

 Gmelin, fig. 43, were collected at Cohasset 

 and Beverly, Mass. A yellowish prodisso- 

 conch exists at the umljos. The left valve of the prodissoconch has a slight sinuosity 

 which corresponds with the compai'atively deeper byssal notch in the right valve, whereas 

 no sinuosity of the left valve and a shallower notch in the right exists in the prodisso- 

 conch of Anomia glabra, PI. xxix, figs. 4-8. This feature is shown in fig. 43, but is 

 more marked in degree in younger specimens. The prodissoconch of A. aculeata is 

 ovally rounded in outline with fine concentric lines of growth and umbos which are nearly 

 central but directed slightly jjosteriorly as in A. glabra. The succeeding dissoconch 

 growth of Anomia aculeata is chalky white, of somewhat irregttlar growth; and not pol- 

 ished or as translucent as is A. glabra. The left valve of the dissoconch does not per- 

 petuate the sinuosity of the prodissoconch valve. This valve is smooth for a brief period 

 and then the ])lications and spines characteristic of the species are commonly introduced. 

 I say commonly, for associated specimens are frequently wanting in the spinous charac- 

 ter, although imquestionably belonging to the same species. The Y\^\\t valve of the dis- 



' Nou-marine Fossil Mollusca of North America, Washington, 1883, p 



Fig. 43. Young Anomia aculeata, tip of left valve , 

 p, prodissoconch, succeeded by early smooth and later 

 spinous dissoconch growth. Cohasset. X GO diam. (Drawn 

 by the author.) 



IG. 



