168 UNITED STATES GEOLOGICAL SURVEY OE TIIE TERRITORIES. 



followed, as deciding what species is to be the type of the genus, as Linnaeus 

 did not so designate any particular species. 



If I am correctly informed, Miiller, in 1776, first adopted Cardiwm after 

 Linnaeus, and, after him, Fabricius in 1780, both of whom cite first under it 

 ('indium echinatum (an Acanthocardium of Gray). As these authors, how- 

 ever, were each only enumerating a local fauna, and not writing a systematic 

 work on the mollusca, we can hardly regard them as fixing the type of the 

 genus; and as Bruguiere, in 1789, gave the genus by his citations nearly the 

 same limits that Linnaeus did, it. seems that we must come down to Lamarck 

 to know what the type of Cardium is. In 1799, the last-mentioned author 

 adopted this genus in his classification of shells, citing Cardium aculeatum as 

 his only example. So, unless some other systematic author that I have had 

 no opportunity to consult, between Linnaeus and Lamarck, may have taken 

 C. costatum or some closely-allied form as the type of the genus, I should 

 think C. aculeatum should be so regarded. 



The genus Cardium, with the limits here assigned it, is, I believe, not 

 certainly known to be older than the Jurassic, in which species of the typical 

 section occur. This subgenus is also represented through the subsequent 

 formations and in our existing seas The Trachycardium and Criocardium 

 groups occur in the Cretaceous, in which the latter attains its greatest devel- 

 opment, while it seems not to have survived the close of that epoch, or at 

 least not to have been represented by decidedly characteristic species, at any 

 subsequent time. The Trac/iycardium section, however, continued on through 

 the Tertiary, and appears to attain its maximum numerical development in 

 our existing seas. The subgenus Cerastoderma is at least as old as the 

 Cretaceous, occurs in the Tertiary, and. according to some conchologists, is 

 represented by twelve to fifteen existing species. Trogidocardium, however, 

 is probably not older than the Eocene, and is at present only known to be 

 represented by two or three living species; while Nemocardium seems to be 

 nearly entirely confined to the Eocene Tertiary, the only Cretaceous species 

 known to me that appears to belong to it, being Cardium subhillanum, 

 Leymerie. 



