376 AXEL A. OLSSON 



frequently strongly flexed towards the right. The umbones are usually wide 

 and flat, the beaks small, inconspicuous, adjacent, and prosogyrous. The 

 ligament is external, usually attached to a narrow nymph, or, in groups 

 with a concentrated hinge, the resilifer may lie somewhat deeper in the 

 hinge plate. Hinge with one or more cardinal teeth, simple or bifid, the 

 lateral teeth may be large and strong or wholly absent. The pallial sinus is 

 usually large and ample, its upper limb sometimes extending across the 

 interior of the valve to contact with the anterior adductor scar, or it may 

 be shorter with a well-rounded end. External surface is smooth with growth- 

 line sculpture only, or it may be more strongly marked with concentric ribs, 

 obliquely incised lines, more rarely with radials. Inner margins of the valves 

 usually smooth. 



Most Tellinas are marine although some species can tolerate fairly 

 brackish water conditions. The largest and finest species of the family are 

 tropical and many species are showy with fine color, usually in tones of red, 

 pink, yellow or violet; northern species have usually more somber tints, 

 often with a chalky surface protected by a coarse periostracum. The shape 

 is variable, some species have strongly flexed valves. 



A satisfactory classification of the Tellinidae remains to be worked 

 out as it should be based on a world-wide study of the shells, both Recent 

 and fossil, as well as on the soft anatomy. In this review of the Panamic- 

 Pacific species, I have followed Dall in placing the genera in two sub- 

 families based on the presence or absence of lateral teeth. This arrange- 

 ment is unnatural as it brings together genera which in other respects do 

 not appear closely related. 



Key to the genera of Panamic Tellinidae 



I. Hinge bearing both cardinal and lateral teeth Tellininae 

 A. Anterior and posterior lateral teeth are strong, equidistant, and placed 

 some distance from the cardinals. 



AA. Valves narrowly to broadly elongated, sublanceolate, inequilateral, the 

 posterior side shorter, narrower, often rostrated and strongly flexed 

 towards the right. 



1. Surface with strong concentric sculpture. 



Genus Tellina 



AB. Valves elongately subovate to subquadrate, the posterior side generally 

 but little flexed. 



2. Valves subovate in shape, both ends rounded, the posterior-dorsal area 

 not defined. Sculpture showing radial riblets on the posterior slope. 



Genus Elliptotellina 



3. Valves subquadrate in shape, the posterior-dorsal area well defined. 



Sculpture formed by strong, often lamellose concentrics, generally en- 

 larged and foliated along the posterior-dorsal margin. 



Genus Pkyllodina 



AC. Valves higher, subovate to subtrigonal in shape. 



a. Posterior-dorsal area well defined, set apart by a ridge or by a marked 

 change of sculpture. 



4. Valves elliptical in shape and marked with strong, concentric sculpture. 

 Posterior side rounded, not extended into a beak. 



Genus Lyratellina 



