1919.] NATURAL SCIENCES OF PHILADELPHIA. 51 



rapidly than the anterior. Ventral margin approximately straight 

 medially, slightly upcurved distally. Umbones low, opisthogyrate, 

 situated a little behind the median line. 



Sculpture. — External surface smooth except for feeble incre- 

 mentals. Faint traces of radial color markings also visible. 



Ligament. — Ligament external, opisthodetic, seated on a narrow 

 and elongated nymph. 



Dentitio7i. — Dentition as in Alacoma virgvniana Conrad. An- 

 terior cardinal of the right valve simple and stout. Posterior bifid 

 and relatively slender. In the left valve a bifid, relatively heavy 

 anterior, and a simple, sub-laminar posterior cardinal. 



Muscle hnpressions and Pallial Line. — Interior faintly striated 

 radially. Anterior muscle impression elongated. Posterior semi- 

 elliptical. Pallial sinus deep, projected more than two-thirds of 

 the distance forward toward the anterior margin, confluent ven- 

 trally with the pallial line for about one-half its length, its dorsal 

 margin, in the right valve, bowed upward perceptibly beneath 

 the umbones; in the left valve, apparently much more nearly 

 parallel to the base. 



Dimensions. — Right valve. Altitude, 12.3 mm. Latitude, 18.2 

 mm. Semi-diameter, 4.3 mm. Left valve, of another individual. 

 Altitude, 12.5 mm. Latitude, 19.5 mm. Semi-diameter, 3.3 mm. 



Type Locality. — Darlington, South Carolina. Duplin Forma- 

 tion. 



Observations. — Macoma carolinensis is the analogue in the Neo- 

 gene south of the Hatteras axis of Macoma virginiana and its sub- 

 species conradi north of the axis. It is rather higher, less 

 flexuous, and more nearly equilateral than either of the former, 

 both by reason of the more nearly central umbones and the greater 

 similarity of the anterior and posterior extremities. The posterior 

 dorsal margin of M. carolinensis has a uniform, fairly steep slope, 

 while that of M. virginiana is slightly contracted directl}' behind 

 the umbones, then obliquely produced at a very low angle. The 

 posterior lateral margin of the former is rounded into the slightly 

 upturned base, that of the latter, obscurely truncated and its ven- 

 tral margin more strongly recurved. The pallial sinus in both spe- 

 cies varies quite widely but that of carolinensis is, on the average, 

 more profound, and more strongly arched beneath the umbones. 



The species is rare in the Duplin and Waccamaw Formations of 

 North and South Carolina. 



