184 PROCEEDINGS OF THE ACADEMY OP 



Genus HELICODISCTJS, Morse. 



Jaw according to Morse, of the only known species, H. lineatus, 

 low, wide, cresentic, ends much attenuated, acute ; cutting margin 



with a median, beak-like projection ; 

 anterior surface without ribs, but 

 covered with striae converging ob- 

 liquely towards the beak-like promi- 



Jaw of H. lineatus. [Morse.] 



nence. 



According to my rule of admitting in the genus Helix only such 

 species as have a ribbed jaw, I am forced to recognize lineatus as 

 a distinct genus. Fig. 27 shows the general arrangement of the 



Fig. 27. 



Lingual dentition of H. lineatus. [Morse.] 



teeth upon the lingual membrane. The characters of the separate 

 teeth are better shown in my plate XIII. , fig. 5. Morse gives 77 

 rows of 12 1 12 teeth, each with 4 perfect laterals. Leidy, in 

 Terr. Moll. U. S., II. 262, fig., gives 13113 teeth, with 5 perfect 

 laterals. The membrane examined by me has 12 1 12 teeth, 

 with 4 perfect laterals. The central teeth have a base of attach- 

 ment very small, longer than wide, with expanded lower angles, 

 and reflected upper margin. Reflection very small, with a stout, 

 short, median cusp, and very short, blunt side cusps, all the 

 cusps with short cutting points. The lateral teeth have a base of 

 attachment three times as wide, and somewhat longer than the 

 centrals, and unsymmetrical by the suppression of the inner, lower 

 lateral expansion ; the upper margin is broadly reflected ; the 

 reflection is short but symmetrical, having two equally developed 

 short, stout side cusps, bearing short cutting points ; the median 

 cusp is stout, long, extending nearly to the lower edge of the 

 base of attachment, beyond which projects slightly the short 

 cutting point. 



The marginals are low and wide, the reflection as broad as the 

 base of attachment, reaching nearly to its lower edge, and furnished 



