178 Bulletin 29 342 



Scapharca Arthurpennelli ', n. sp. 

 Plate 29, Figures 9, 10 



This is probably the form which Gabb had in mind when he 

 spoke of the abundance of A. Pennelli; but neither Gabb's speci- 

 mens nor ours correspond to his description of that species. This 

 shell is not an Anadara since it is inequivalve. It is not very 

 oblique, and the left valve is handsomely beaded, though the 

 ribs of the right valve are smoother, the sculpture being some- 

 what discrepant. The ribs number thirty; the shell has a general 

 resemblance to the round, inflated, typical Argi?ia campechensis 

 Dillwyn; but is more solid and the cardinal area is too wide for 

 Argina except in a few individuals. There are several grooves, 

 part of which extend in front of the beaks. The hinge is wide 

 with long, irregular teeth which somewhat resemble those of 

 Argina but are less abnormal. The species is intermediate be- 

 tween Scapharca and Argi?ia. Many of the specimens, both ours 

 and Gabb's, are characterized by a heavy callus at the anterior 

 basal margin. Those individuals which have this to an extreme 

 form appear pathologic. Length 35, altitude 32, semidiameter 

 15 mm. Besides hundreds of our own shells we have a large 

 number loaned by the American Museum, collected at Bulla 

 River (see sketch map). 



Area tolepia Dall appears to be similar to A. Arthurpennelli 

 but is described as thin, the sculpture of the right valve is differ- 

 ent from ours and the cardinal area is so narrow behind the beaks 

 that Dall placed the species in Argina. Our shell is remarkably 

 heavy and thick for its size and the cardinal area is too wide for 

 Argina. 



Localities. — (Exp'd '16) Bluff 2, Cercado de Mao (compara- 

 tively scarce); Bluff 3 (exceedingly abundant, all ages and sizes). 



Scapharca (Argina) tolepia. Dall 



Area pexata Guppy, Fide Dall, Geol. Mag. London, p. 443, 1874; Not 



of Say 1822. 

 Scapharca {Argina) tolepia Dall, Trans. Wagner Inst. Sci., vol. 3, pt. 4, 



p, 649, pi. 33, figs. 7, 8, 1898. 



