THE NAUTILUS. 143 



Atrina, characterized by the absence of any slit in the umboiml part 

 of the shell, such as is found in all the typical Pinnas. 



Atrina oldroyUi n. sp. 



Shell solid, heavy, blackish-gray, subtriangular, rather inflated ; 

 umbonal end slender (somewhat defective in the specimen); hinge 

 margin straight ; ventral margin contracted in front, convexly arcuate 

 behind; posterior margin arched ; exterior smooth, except for more 

 or less concentric wrinkling on the ventral side and numerous rather 

 fine imbricate elevated ridges (about 38) radiating from near the 

 umbo on the dorsal and middle portions of the valve, not extending 

 to the ventral surface and obsolete over the distal fourth of the valve ; 

 the scales or spines are worn off, but appear to have been numerous 

 and small ; interior of a livid dark olive gray, with a lurid iridescence 

 over the visceral area, the ventral edge of which extends in a zigzag 

 line almost directly anterior from the ventral edge of the rather 

 small adductor scar, leaving more than a third of the ventral surface 

 of the inside of the valve exterior to the visceral area. Length of 

 ventral margin 238 ; of dorsal margin 175 ; of the distal margin 156; 

 maximum diameter of the valves 63 mm. Length of the visceral 

 area from the umbo 172 mm. The byssus is quite small and of a 

 dark blackish-brown color. 



The form of the visceral area, which in these shells is generally 

 regarded as a pretty constant character, is entirely different from that 

 of any of the other described Pacific const species. In the form 

 which, as described, comes nearest to A, oldroydii (_A. tuber culosa^ 

 has the posterior margin of the visceral area forming a straight line 

 from the dorsal nearly to the ventral margin of the valves. 



The present species appears to be an analogue of our Atlantic 

 coast A. fe/rata Sovverby, but as regards the exterior characters prob- 

 ably submits to a variation which only the study of a larger number 

 of specimens will enable us to determine. 



Though not a particularly handsome shell, this is one of the most 

 notable among the many additions made to the mollusk fauna of Cal- 

 ifornia in recent years. 



ALASMODONTA MARGIXATA, SAY, AND A. TRUNCATA, WKICIIT. 

 In his " Synopsis of the Naiades," Mr. Simpson says the former is 

 from " Lower St. Lawrence, southward in streams draining into the 



