140 



Family Mesodesmatidce. 



Mesodesma deauratum (Turton). 



Mactra deaurata, Turton (1822). 



Mesodesma Jauresii, De Joannis (1834) ; and Gould (1841). 



Ceronia deaurata, H. and A. Adams (1858) ; and Gould (1870). 



Under the generic name Ceronia, both M. deauratum, and M. arctatum 

 (Conrad) are said to be rare on the fishing banks off Sable Island, by Willis 

 in his latest (1863) list of ISTova Scotia shells. On the authority of the late 

 Dr. Gould, Dr. R. Bell referred to M. arctatum the specimens that he col- 

 lected on the south shore of the lower St. I^awrence below Green Island, in 

 1858. On the other hand, Dr. Packard identified with M. Jauresii (a 

 synonym of AI. deauratum) all the specimens that he found thrown up 

 abundantly on beaches near Caribou Island, in 1860. And, following Dr. 

 Packard, all the specimens collected at Lit^rle Metis in 1867, and off Magpie 

 village (on the north shore of the St. Lawrence) in 1871, by the writer, 

 and at Tadoussac by Sir J. W. Dawson, have been referred to xW. deauratum,. 

 The writer has long been under the impression that there is only one species 

 of Mesodesma in Canadian waters, and that the distinction between M. arcta- 

 tum and M deauratum can scarcely be satisfactorily maintained. Young or not 

 quite adult specimens, with the valves covered with a yellowish and sometimes 

 slightly iridescent epidermis agree in that respect with the description of 

 M. arctatum ; whereas adult and aged specimens, with only a narrow strip 

 of pale ashen gray epidermis around the ventral margin, correspond better 

 ■with that of M. deauratum. Judging by the figures in the second edition of 

 Dr. Gould's Invertebrata of Massachusetts, the principal difference between 

 these two forms would seem to be that in M. arctatum the short posterior 

 end is so abruptly subtruncate that the beaks are almost terminal ; whereas 

 in M. deauratum, the same part of the shell is somewhat produced. In this 

 particular, all the Canadian specimens that the writer has seen, agree better 

 with the figure of M. arctatum than with that of if. deauratum. 



In a letter dated Feb. 18th, 1901, Dr. Dall writes as follows on this point. 

 " In regard to the Mesodesmas Stimpson recognizes three forms : 



arctatum,, Conrad (1831). 



cmer^a, Stimpson (M.S.) ' 



deauratum,, Turton (1822) 



more rounded. 



flattened. 



longer and more angular. 



yellowish. 



thin. 



do. 



narrower. 



