166 UNITED STATES GEOLOGICAL SUEVEY OE THE TERRITORIES. 



of subordinate groups, differing more or less from the typical form. These 

 may be summarily characterized as follows: 



1. cardium, Linnaeus (typical, zz Acanthocardium, Gray). 



Shell subglobose, rather thick, nearly equilateral; cardinal teeth 

 less strongly developed than the lateral; posterior gap nearly or cpiite 

 closed; surface with radiating, more or less spinous, or tubercular 

 costse. — Cardium aculeatum x Linn. 



2. pectunculus, (Adanson). Stoliczka ; but not H. and A. Adams and 



others. 

 Shell nearly as last, excepting in having smaller, more crowded, 

 and smoother costse. — Cardium Asiaticum, Lam 



3. TRACHYCARDIUM, Moich. 



Shell thick, subglobose, or nearly vertically suboval ; hinge 

 arched, with cardinal and lateral teeth strong ; surface with numerous 

 squamous, radiating costse ; posterior gap narrow, and often with 

 more or less dentate margins. — Cardium alternatum, Sow. 



4. criocardium, Conrad. 



Shell much as in the last, excepting in having spines or 

 tubercles arranged in the furrows between the radiating costse ; and 

 the posterior margins closed, and, like the free edges all around, 

 merely crenate within. — Cardium dumosum, Conrad. 



5. tropidocardium, Roemer (Cardium proper, of most authors). 



Shell rather thin, gibbous, cordate, nearly erpiilateral and erect, 

 with hinge-line nearly or quite straight ; posterior gap usually 

 distinct, with margins dentate or serrate ; cardinal teeth moderate ; 

 laterals compressed ; surface ornamented with elevated, carinate, more 

 or less spinous, radiating costaj. — Cardium costatum, Linn. 



6. CERASTODERMA (Poli), Morcll. 



Shell transversely or more or less obliquely ovate-subcordate, 

 convex, inequilateral; surface with 1 hick, closely-arranged, radiating 

 costse ; valves closed, and nearly or quite smooth posteriorly : hinge- 

 teeth generally well developed. — Cardium edule, Linn. 



