1911.] NATURAL SCIENCES OF PHILADELPHIA. 363 



becomes nodulose, and the intervals are crossed by their lamellae. 

 In the right valve the ribs are smooth in the middle field, the intervals 

 concave, with concentric lamellae towards the beaks. At both ends 

 the ribs are wider and divided by a groove, as in the other valve. 

 In both valves the anterior ribs have a shallow sulcus, the posterior 

 ones a distinct narrow furrow. The ligament area is narrow, lanceo- 

 late, marked with two or three grooves diverging at a wide angle 

 The teeth are very fine and close, about 70 in a shell 36 mm. long 

 Interior radially striate, the margin crenulate as usual. 



Length 36.0, alt. 20, diam. 18.2 mm. 

 " 31.8, " 18, " 13.5 " 



This is an abundant Ark in the Gatun bed. It is well-characterized 

 by the rather narrow shape, grooved posterior and shallowly sulcate 

 anterior ribs, while the ribs of the median part are simple in the right, 

 granulous in the left valve. 



Glycymeris carbasina n. sp. PI. XXVIII, fig. 9. 



Shell of the usual subcircular shape, solid, equilateral, with slightly 

 prosogyrate beaks, scarcely separated by the very narrow ligament- 

 area. Sculpture of many very low radial ribs equal to the intervals, 

 with over the whole a minute sculpture of radial cut by equal concen- 

 tric striae, forming a regular granulation by their intersection. On 

 each rib (to the middle of the interval) there are five or six radial 

 granule-threads. The antero-dorsal and postero-dorsal areas have 

 no major ribs, but there are fine radial riblets, the intervals of which, 

 being crossed by the concentric striae, appear to have a single series 

 of punctures in each. The hinge is rather wide. Tooth-row of 10.12 

 teeth meeting V-like in the middle, where they are invaded by the 

 ligamental area in the largest specimen seen. Inner margins of the 

 valves crenulate throughout, the "teeth" of the basal portion very 

 deeply cut, narrow, with a concavity in each. 



Length 18, alt. 16, semidiam. 5 mm.; 66 crenulations in the margin,, 

 hinge teeth 10.11. 



Very closely related to G. jamaicensis Dall, of the Bowden Oligocene,, 

 but that has much smaller, more numerous hinge-teeth in shells 

 of the same size, the anterior contour is more irregular, and the 

 fluting of the basal margin is far less deep. The external sculpture 

 is practically identical in the two forms. The recent Antillean G. 

 lineata (Rve.) is extremely close to G. carbasina, but in the latter the 

 marginal fluting is far more emphatic. We do not know that the 

 fossil form ever attains nearly the size of G. lineata, of which it is 

 probably a direct ancestor. 



