244 AXEL A. OLSSON 



or Cardiinae, however, are absent: Trackycardiinae. — in this group, the 

 shell is solid, strongly ribbed, the sides and crests of the ribs ornamented 

 with spines, nodes and scales, the posterior and ventral margins of the 

 valves serrated or fluted by the ends of the ribs; Fraginae. — solid or thick- 

 walled shells, usually quadrate to rhombic in shape, the posterior slope 

 strongly flattened or depressed, bordered by a sharp, umbonal angle, the 

 surface of the ribs smooth or noded, their interspaces cross-threaded, the 

 margins of the valves not serrated; Proto cardiinae. — the shell usually thin, 

 round, or quadrate in shape, the posterior surface set apart by an umbonal 

 ridge (often fringed), and by a sharp change of sculpture, the radial in- 

 terspaces often with scattered pustules; Laevicardiinae. — shell ovate to egg- 

 shaped, often thin, smooth, glossy, or with small flat riblets, the posterior 

 slope weakly defined, the valve margins smooth or crenulated. 



Key to genera of Panamic Cardiidae 



I. Surface smooth, often highly polished and with fine or obscure, radial 

 lines or riblets formed within the inner layer, and which produce fine 

 crenulations along the inner ventral margins of the valve. 



Genus Laevicardium 



II. Sculpture of the surface stronger, usually with strong, radial ribs or 

 riblets. 



A. Sculpture similar over the whole surface, the lateral ribs gradational 

 with those in the middle. 



1. Length of shell greater than its height. Beaks anterior of the middle, 

 the margins of the valve not closing tightly but with an open gap at 

 both ends. 



Genus Papyridea 



2. Height of the shell is greater than its length. Valve margins closed, not 

 gaping. 



2a. Posterior side of the umbone with a strong, angled keel, its slope flat- 

 tened, depressed, and truncated. 



Genus Trigoniocardia 



3. Posterior-umbonal slope merely rounded. 



4. Ribs low, round or flat, smooth, or with weak nodes. 



4a. Shell ovate and strongly convex. Surface color white, generally mottled 

 with brown and covered by a heavy, dark-brown periostracum. 



Genus Mexicardia 

 4b. Shell oblong-ovate, moderately convex, the ribs low, close, smooth, or 

 weakly imbricated. 



Genus Acrosterigma 



5. Ribs higher, rectangular, between deeply grooved interspaces, ornate 

 or adorned with sharp, imbricated scales tubercles, or spines. 



Genus Trachycardium 



B. Sculpture differentiated into two, distinct areas, the smaller, posterior 

 area set apart from that on the middle by a ridge or fringed keel as 

 well as a marked change of detail. Substance of shell usually thin. 



6. Shell longer than its height, the posterior side contracted, and slightly 

 gaping at the end. Posterior area setoff by a raised lamina. Hinge 

 without lateral teeth. 



Genus Lophocardimn 



