] 76 Dall — Diagnoses of New Sj)ecies of Mollusks. 



Ischnoc'iiton biarcuatus sp. nov. 



Animal about 18 mm. long and 7 mm. wide (in the dry state); girdle 

 narrow, with very small, close-set, more or less imbricating, brownish 

 scales: valves rounded evenly above, only the lateral areas distinct; an- 

 terior valve with 7 or 8, median with 1, posterior valve with 11 slits; in- 

 terior of valves rose-pink; exterior ashy, marbled with lilac and brown, 

 an obscure lilac median line on the medial valves; sculpture of undivided 

 central areas formed by two sets of arcuate radiations crossing each 

 other obliquely and with the inter-reticulations impressed or punctate, 

 so that an irregularly zigzag effect is produced by the arrangement of 

 the punctations; lateral areas irregularly concentrically vermiculate, the 

 spaces between the elevated ridges deeply minutely punctate, with some- 

 what of a zigzag effect here also; the sculpture of the anterior valve re- 

 sembles that of the lateral areas; of the posterior valve the mucro is low, 

 sub-central and inconspicuous, the central area sculptured like that of 

 the medial valves, the posterior area like the anterior valve; the sutural 

 plates are quite short and the sinus smooth and wide. There is no notice- 

 able mucro to the medial valves. 



The peculiar sculpture of this species separates it from those already 

 described from this region. In a general way it recalls the very young 

 of /. magdalenensis Hinds. The type is No. 109,308, U. S. National 

 Museum. 



