INVERTEBRATE RALiEONTOLOGY. 215 



2. Procardia, Meek. 



a. (= ? BucardincB. (sp.), Ag.) — Shell short-subtrigonal, gibbous, 

 with beaks elevated and strongly incurved or subspiral ; anterior side 

 very short, truncated, and impressed like a large cordate lunule, with 

 a smaller, deeper, inner lunule under the beaks ; cardinal margins 

 strongly incurved and without a false area; surface provided with 

 concentric furrows, crossed on the sides by distinct radiating costse. — 

 Pholadomya Hodgii, Meek. (Cretaceous.) 



b. (— ? BucardincB, Ag., and Cardissoides and Trigonatce (sp.), 

 Ag.). — Shell short-subtrigonal, generally high and very gibbous ; 

 anterior side often more or less truncated, and sometimes gaping ; 

 posterior generally distinctly gaping ; beaks elevated ; muscular and 

 pallial impressions sometimes well defined ; false area either defined 

 or wanting; surface with a few, often tubercular, radiating costse on 

 the flanks, varying much in prominence. — P. exaltata and P. cardiss- 

 oides, Ag., and P. nodulifera, Minister. (Jurassic.) 



I am not quite sure that any of the species figured by Professor Agassiz, 

 in his monograph, under the group BucardincB, belong properly to the typical 

 section of the subgenus Procardia. The type of the latter, as may be seen 

 by our figures, is a very peculiar shell. Its remarkably short, trigonal, 

 gibbous form, elevated, subspiral beaks, and truncated front, with a deep 

 inner lunule under the beaks, altogether present so unusual a combination of 

 characters, that, in first publishing a preliminary description of it, I was left 

 in considerable doubt in regard to the group or section to which it properly 

 belongs, and placed it provisionally, with a mark of doubt, in the genus 

 Isocardia. At the same time, however, I intimated that its general form, 

 distiuct double lunule, and radiating costse, indicated quite different affinities, 

 and suggested for it the provisional name Procardia, stating that it seemed to 

 belong to the same group as the so-called Cardium decussatum of Sowerby. 



As figured by Sowerby, Mantell, and Goldfuss, Sowerby's species men- 

 tioned above, certainly appears in no respect more than specifically different 

 from the type of the foregoing section Procardia. But the form figured by 

 Professor Agassiz under Sowerby's name P. decussata, is not only quite different 

 in nearly all respects from the same, but differs so widely from the typical 

 form of that species, as illustrated by the other authors already mentioned, 



