OAR ''< N M-KROUS AGE. 31 



Prof. Koninck described tliis genus as being cipiivalve nn error into which he 

 doubtless led b\ the distortion of his specimens. IV'f. McCoy subsequently 

 des< ribed it as inequi\ ;d\e. which is undoubted!) correct, as we know from the 

 stiuh of our American species. As remarked b\ 1'mf. Koninck, the species oft in 

 dosi 1\ reseinlile Mi/til n.t and /' n/n (Hrug. not Adanson) in form ; hut the inequality 

 of the vahes, and the broad striated cardinal area, will distinguish tlieni from tin- 

 first and the absence of deep \ertical cartilage furrows in the hinge area, from the 

 latter. They seem to he e\en more closely allied to the fresh-water genus Dreiatena, 

 \\ ith which some of the species agree c\actl\ in form, excepting the slight inequality 

 of the \al\es. They also sometimes possess an internal shelf or septum in each 

 beak, as we see in that genus. \Ye ha\e likewise discovered that the shell struc- 

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



!>,, 'I.-M i, 'nl, i and Ari'i -iili'lu. 



\\ 'ere it not for the broad striated area, and the inequality of the valves, we 

 would certainh be inclined, from all the facts, to place this genus in the Dreissenidce, 

 instead of the J //////</. The prismatic structure of which we speak, settles the 

 question in regard to these shells being distinct from the family Mi/tilidte ; but as this 

 structure occurs Ixith in the l)r> /.<.- n!<ln- and the .-I o'< /,V/,p, it is not alone so conclusive 

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

 howe\ er, may yet enable us to decide this point, since in the Aviculidte the inner layer 

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

 in the Dr> !.<* a'ahr 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 diffi- 

 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, in 

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

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

 character, and broad cardinal area indicate. 



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

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

 diameters. 



Shell rtructure of Myalina angvlata, magnified 350 diuneton. 



The genus Myalina 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- 



