NO. 1 SOOT-RYEN : THE FAMILY MYTILIDAE 93 



Incrustation more uniform or the thickened part 

 smooth or with a shallow divaricate pattern 4 



4. The protruding incrustation or the ridges indicat- 

 ing the termination are crossed, generally with 



that of the right valve the lowest Myoforceps Fischer 



Incrustation more uniform and alike on both 

 valves without crossing projecting parts 5 



5. Projecting part of incrustation generally rather 



long or ending in a median spine, smooth Labis Dall 



Projecting part of incrustation, if present, short, 



surrounding posterior margin of valve Leiosolenus Carpenter 



Subgenus STUMPIELLA new subgenus 



Diagnosis: Shell with a triangular median area covered with a solid in- 

 crustation and bounded by a dorsal and a rather distinct ventral furrow. 

 The incrustation has one dorsal and one ventral ridge terminating pos- 

 teriorly in sharp angles, and is furnished with irregular wrinkles between 

 the ridges, at least in the median part. The protruding incrustation is 

 closed behind and furnished with one dorsal and one ventral opening 

 connected by an internal channel. 



Type of subgenus: Lithophagus calyculatus Carpenter 1856. 

 Remarks: The description given by Carpenter of his new species does not 

 agree in all details with the three specimens at hand, but the peculiar 

 projection of the incrustation seems to be such a good character that I 

 think these shells can be referred to Lithophaga calyculata safely. 



Lithophaga (Stumpiella) calyculata (Carpenter) 1856 

 Plate 10, figs. 61-63 



Lithophagus calyculatus Carpenter, Catalogue of the Reigen Collection 

 of Mazatlan Mollusca, 1856, pp. 124-125 (non calyculatus Hertlein 

 and Strong 1946 = plumula Hanley). 

 Type loc: Mazatlan, Mexico. 

 Holotype: British Museum. 



Remarks: Shell rather high with a distinct dorsal angle, umbones near 

 anterior margin, which is truncately rounded; ventral margin nearly 

 straight, posterior margin concave, bounded by the two sharp angles 

 formed by the protruding ridges of the incrustation. Seen from behind, 

 the incrustation forms an oval closed area with slightly elevated median 

 ridge and circumference ; seen from the dorsal or ventral side, the incrus- 

 tation has openings for a roundish channel passing through it. The dorsal 



