1894. PROCEEDINGS OF THE NATIONAL MUSEUM. 103 



Length, 43; heiglit, 30; diameter, 2(5 mm. in the adult. 



This is apparently a rare species. I have never seen a perfectly 

 fresh specimen. It can be distinguished from any other Atlantic spe- 

 cies by its short, subequal, lateral teeth, rostrate shape, and obsolete 

 l^allial sinus. The crennlations of the teeth are very feeble and, in 

 worn specimens, sometimes invisible; but this is a character which 

 varies much in individuals, as can be observed in any good series of 

 G.cuneatus. Tlie shell is much heavier than G. mendiciis, and has the 

 lateral teeth more unequal, the anterior lateral being strongly hooked. 

 The pallial line has no reentrant angle, but a recess is formed by the 

 base of the adductor scar and the vertical extension of the pallial 

 impression. The epidermis is straw yellow. I have observed no bead- 

 ing along the lines of growth, and no sculpture, on the posterior slope, 

 except incremental lines, on any of the specimens I have examined. 

 The shell varies a good deal in height relative to its length, and the 

 posterior end may be flexed upward or downward or produced horizon- 

 tally. It is perceptibly rostrate. 1 have called attention to the fact 

 that G. cuneatus has a rostrate variety, of which the outline simulates 

 that of G.^fiexvosii.s, and must now add a peculiar variety of G. jiexuosus 

 which tends in the opposite direction. 



GNATH()I)()x\ FLEXrOSl'S var. I'ETITIANUS, 1) a 1 1 . 

 Plate VII, fij.-. 5. 



In this variety the shell has a height of 30 and a length of oO-.j mm. 

 compared with a height of 1*0 and a length of 43 mm. for the typical 

 Jfe.rnosiis; the posterior slope is convex and the rostrum short, rounded, 

 and bentdownward,thepallialIine hasafaintsinuation, and the "hook" 

 on the anterior lateral is obsolete. The diameter of the shell would be 

 about 21 mm. The valve is lighter than any specimens of (^.Jiexuosus 

 which have come to my notice and considerably more swollen. It 

 is possible that it may represent a distinct species, but this can not be 

 determined without a good series of fresh specimens, A single some- 

 what worn left valve was obtained by the Mexican geographical com- 

 mission at Vera Cruz, and is now in the Xational Museum (No. 57G08a). 



I have referred it to G. Jiexuosus on account of the shoit lateral 

 teeth and feeble pallial sinus; if additional material should prove it to 

 be distinct, the varietal name now given may be taken as specific. No 

 indication of external sculpture except incremental lines is visible; the 

 epidermis is absent from the specimen. 



G. jiexHosus possesses more constantly than any other species a 

 character occasionally found in each of them, namely, the presence 

 at the anterior border of the cartilage pit of a thin accessory lamella 

 between the pit and the deltoid cardinal tooth of the left valve. This 

 lamella, when j)erfect, looks like an additicmal cardinal tooth, and is 

 always best developed in the left valve, but it is usually more or less 

 absorbed or even absent. It is common to all the Mactridw. The mar- 



