124 THE NAUTILUS. 



ment of the apertural lamella? in these three species is fundamentally 

 the same. Their number, position and character are the same in 

 all. The differences, which are, however, specifically characteristic, 

 are only in the degree of development. 



In armigera, the principal parietal lamella is much shorter, less 

 oblique and its anterior end is less curved and truncated than that of 

 crassilabris, the palatal folds are all less developed, and the spaces 

 between them consequently greater ; the upper extremity of the basal 

 fold is less deflected and scarcely noticable externally. In both 

 these species, there is a marked tendency to develop an additional 

 fold in connection with the central palatal giving it a V-shape, which 

 is quite noticeable from the exterior. 



In wheatleyi, the position of the principal parietal lamella is more 

 like that of armigera than of crassilabris, being, taken as a whole, 

 less oblique and the anterior end is less bent forward than in the 

 latter species, but the posterior extremity is more prolonged than in 

 armigera, and is continued for a short distance parallel with the 

 suture ; the basal palatal is much wider than in either of the other 

 species and is abruptly contracted at its base, while the upper ex- 

 tremity is deflected as in crassilabris ; the second palatal fold is re- 

 markably developed, rising abruptly near the base of the whorl ; it 

 runs back for a short distance parallel with the base, and curves up- 

 ward, terminating just below the superior carina, having much the 

 shape of a boomerang. The third palatal is only slightly oblique, 

 the lower end is bent backwards and close to the upper extremity is 

 a small knob, not sufficiently differentiated from it to be called a dis- 

 tinct denticle, but evidently similar to the accessory fold noticed in 

 the other species ; the upper extremities of this fold and of the basal 

 palatal are about on the same level. The fourth palatal is situated 

 directly under the superior carina and is a short, straight, transverse, 

 club-shaped fold, the posterior extremity being the thickest. 



EXPLANATION OF PLATE VII. 



Figs. 1-3. S. armigera Say, Mohawk, N. Y. 



Figs. 4-6. S. crassilabris (Type), Hamtramck, Mich. 

 Figs. 7-9. S. wheatleyi Lea, Princeton, Ala. 



