ART. 12 A NEW MOLLUSCAN GENUS FKOM CHINA KIRK 6 



deposits of the Tung Kou District, Szechwan, China. Seven speci- 

 mens are available for study. The preservation is excellent. The 

 specimens are as a rule somewhat crushed, but the ones chosen for 

 illustration are relatively free from distortion. 



The specimen used for measurement and description is the smallest 

 in the lot. Its maximum length, measuring from the beaks to the 

 posterior margin, is 7 cm. The maximum height is 3.25 cm. and the 

 maximum breadth 3.4 cm. The largest specimen gives the following 

 measurements : Length, 8.2 cm. ; height, 3.6 cm. ; and breadth (some- 

 what compressed), 3.7 cm. 



The shell is thick, as in the other genera of the Megalodontidae. 

 In a medium-sized individual the shell has a thicloiess of 9 mm. near 

 the umbones and posterior to the hinge plate. The shell is heaviest 

 along the line corresponding to the angular shoulder on the exterior. 

 From this line of maximum thickness the shell thins gradually 

 to the margins. The surface is marked by strongly developed irregu- 

 lar concentric growth lines. These are especially well developed on 

 the dorsal surface and in the posterior portion of the shell. The 

 lunule is small and inconspicuous. There is a poorly defined 

 escutcheon, which consists merely of a somewhat flattened area with- 

 out definite boundaries on the posterior side. 



The shell is subrhomboidal in outline, and as indicated by the meas- 

 urements and figures is long and low. The hinge line is about two- 

 thirds the length of the shell and is gently arcuate. The anterior 

 margin is short and forms an acute angle with the hinge. In the 

 type specimen the anterior shell margin incurves abruptly just below 

 the beaks, forming a narrow deep pit. This pit is lacking in other 

 specimens. The posterior margin is sharply curved. The ventral 

 margin is almost straight or slightly sinuous, curving upward at the 

 ends to meet the anterior and posterior margins. The greatest 

 dimension of the shell is the line running obliquely from the beaks 

 to the posterior margin. The beaks are sharply differentiated and 

 large. They come almost into contact in the anterior portion and 

 curve forward. The umbonal ridge in the anterior portion of the 

 shell is developed as a sharp keel. Posteriorly the ridge is more 

 rounded but persists as a strongly developed angular shoulder. 

 From this ridge the shell drops abruptly to the dorsal margin. To 

 the posterior and ventral margins the shell curves gently, while to 

 the anterior margin there is a steep pitch amounting to incurvature 

 in some individuals. 



The hinge plate is wide and heavy. Along the dorsal margin it 

 is gently arcuate, following the contour of the shell margin. In the 

 specimen prepared to show the internal structures there are eight 

 well-developed cardinal teeth. The tooth nearest the dorsal side 

 might with almost equal propriety be classed as a lateral, as there is 



