CARBONIFEROUS AGE. 31 



Prof. Koninck described this genus as being equivalve — an error into which he 

 was doubtless led by the distortion of his specimens. Prof. McCoy subsequently 

 described it as inequivalve, which is undoubtedly correct, as we know from the 

 study of om- American species. As remarked by Prof. Koninck, the species often 

 closely resemble Mytilus and Perna (Brug. not Adanson) in form ; but the inequality 

 of the valves, and the broad striated cardinal area, will distinguish them from the 

 first — and the absence of deep vertical cartilage furrows in the hinge area, from the 

 latter. They seem to be even more closely allied to the fresh-water genus Dreissena, 

 with which some of the species agree exactly in form, excepting the slight inequality 

 of the valves. They also sometimes possess an internal shelf or septum in each 

 beak, as we see in that genus. We have likewise discovered that the shell struc- 

 ture is prismatic, as seen under a high magnifier, by transmitted light, as in the 

 Dreissenidce and AvicuUdce.. 



Were it not for the broad striated area, and the inequality of the valves, we 

 would certainly be inclined, from aU the facts, to place this genus in the DreissenidcB, 

 instead of the AvicuUdcs. The prismatic structure of which we speak, settles the 

 question in regard to these shells being distinct from the family Mytilidae ; but as this 

 structure occurs both in the Dreissenidce and the AvicuUdce, it is not alone so conclusive 

 in regard to the relations of these shells to these two families. Further examinations, 

 however, may yet enable us to decide this point, since in the AvicuUdce the inner layer 

 of the shell is not prismatic, but pearly, and the outer layer only is prismatic ; while 

 in the Dreissenidce there is no pearly interior — the inner layer being prismatic, as 

 are the succeeding portions, excepting the very exterior. As the nacreous portion 

 of shells is most frequently destroyed during the process of fossilization, it is difii- 

 cult to determine very satisfactorily whether it was really the inner or outer layer 

 in which we saw the prismatic structure — the shell being very thin, and apparently 

 more or less exfoliated in the specimens examined. Our impression, however, is 

 that it was the outer layer, or at any rate not the innermost ; which would favor 

 the conclusion that these shells belong to the Avicididce, as their inequivalve 

 character, and broad cardinal area indicate. 



The annexed cut shows the prismatic structure as seen in a fragment of MyaUna 

 angidata, placed in Canada Balsam, under a magnifying power of about 350 

 diameters. 



Shell structure of Myalina angulata, magnified 350 diameters. 



The genus MyaUna seems to have been introduced during the latter part of 

 the Devonian epoch, or soon after the beginning of the Carboniferous, and pro- 

 bably attained its maximum development during the deposition of the Coal Mea- 



