PHYLOGENY OF THE TELECYrODA. 



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 diffei-ence in the structure 

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

 that a thin j^rismatic layer exists in the right valve of young Pecten and in the right 

 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 Car2)enter, in his second paper (1818), on shell structure, stated that a thin external 

 prismatic layer exists in Anomia epMjppium; but is early worn oif. In this connection, 

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

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

 asci-ibes the loss of right valves to their fragility owing to their prismatic composition. 

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

 of the Museum of Comparative Zoology 

 have both valves equally j^reserved. A very 

 thin layer of prismatic tissue exists in the 

 right valve of Anomia epliippium, as ob- 

 served in specimens kindly 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 sidi-nacreous 

 tissue, with the addition of porcellaneous tis- 

 sue around the muscular insertions as in An- 

 omia glabra. The diftei-ence in the structure 

 of the right valve of Anomia glabra makes 

 a good specific distinction, which separates 

 that species clearly from A. ej)hip2)ium, 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 umbos. The left valve of the prodissoconch has a slight sinuosity 

 which corresponds with the comparativel}^ 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. xxrx, figs. -4-8. This feature is shown in fig. 43, but is 

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

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

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

 growth o? Anomia aculeata is chalky white, of somewhat irregular 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 pi'odissoconch valve. This valve is smooth for a brief period 

 and then the plications and spines characteristic of the species are commonly introduced. 

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

 ter, although unquestionably belonging to the same species. The right valve of the dis- 



' Nou-mariue Fossil MoUusca of North Amei'ica, Washingtou, 1883, p. Hi. 



Fig. 43. Youug Anomia aculeata, tip of left valvo , 

 p, prodissoconch, succeedeil by early smooth and later 

 spinous dissoconch growth. Cohasset. XGOdiam. (Drawu 

 by the author.) 



