204 MESSES. J. C. MELYILL AND E. STANDEE ON 



umbones to the ventral margin ; rays much elevated, about eleven 

 in number, broader towards the ventral margin, nodulous where 

 the rude concentric striae cross them. "Within, sligbtly nacreous, 

 showing, owing to its extreme tenuity, the precise obverse of 

 the external sculpture ; hinge with an elongate trigonal " fossa," 

 and a marginal " lamina " in either valve. 



The great thinness of the substance causes the shell often to 

 be woru away at the umbones ; this is owing to incessant friction 

 in opening and shutting the valves. Here we have the typical 

 Fholadomya^ a more appropriate cognomen than which could 

 not have been possibly coined, for the resemblance to a large 

 PJiolas (e. g. P. costata, L.) is very striking. Many extinct forms 

 approach the recent shell both in size, facies, and ornamentation, 

 and several even exceed it in beauty and magnitude. These are 

 mainly Secondary and Tertiary fossils, especially abounding in 

 the Liassic or Jurassic rocks. In the majority of instances, owing 

 to the shell-substance having been originally so thin, tbey are 

 mere casts, but are exact replicas of their former analogues in 

 all salient particulars. 



In 1842* Agassiz subdivided the genus into various sections, 

 considering the presence or absence of a posterior area (" aire 

 cardinale circonscrite ") of primary consequence. Thirty-four 

 years later, Meek, in 1876 1, considered that only two divisions 

 were worthy of establishment, viz. : Plioladomya (type, as before, 

 P. Candida, Sow.), distinguished by an elongate insequilateral 

 form, gaping at both extremities, and roundly truncate in front ; 

 and Procardia, to contain the short, gibbous, subtrigonous, ex- 

 tremely truncate, and often elevated forms, to which the greater 

 proportion of the fossil species belong. These now exceed two 

 hundred in number. 



Turning again to the recent forms, the second, discovered only 

 a few years ago, is P. Loveni, Jeffr., of the typical section of 

 the genus, and we consider it here w^orth while reproducing 

 Dr. Grwyn Jeffreys's original description, so as to place it side by 

 side with that of our new species, to which, however, it does not 

 bear much near resemblance. This is a remarkably interesting 



* L. Agassiz, Etudes critiques sur les MoUusques fossiles : Neucbatel, 

 1842-45. 



t A Eeport on the Invertebrate Cretaceous and Tertiary Fossils of the 

 Upper Missouri country : U.S. Geol. Survey of the Territories (Meek), 1876. 



