XXX CLASSIFICATION. 



mwi) sometimes shows longitudinal riblets, and hence has been in- 

 cluded in both divisions of the key. The other subgenera are 

 strongly characterized, natural groups, easily distinguishable in the 

 recent and fossil faunas. 



I. Surface distinctly ribbed or striate longitudinally, at least near the 



apex. 



a. Five-angled, at least apically, Entalina, p. 131. 



a 1 . Not pentagonal. 



b. Apical slit on concave side ; coarsely striate or ribbed, 



Heteroschisma, p. 61. 



b l . Close, fine, even, deeply engraved striae near apex or 



throughout, section circular or nearly so, white or whitish ; 



mainly rather small, Graptacme, p. 85. 



6 2 . Decidedly polygonal or ribbed, at least near apex; slit 



short and on convex side, or wanting, 



Dentalium s. s. t p. 1 ; Antalis, p. 37. 



b 3 . Circular, solid, becoming striate toward apex, which is 

 simple, notched, or with a supplemental tube, 



Antalis, p. 37. 



5 4 . Large, with many riblets, the apex simple or with a slit or 

 row of holes on the convex side, Fissidentalium, p. 63. 

 ft 5 . Small, thin, rapidly tapering, compressed between convex 

 and concave sides ; riblets very low and slight ; see Sec- 

 tion II, Compressidens, p. 123. 



II. No longitudinal sculpture. (See also Siphonodentalium,ip. 135). 



a. Cavity of shell bicostate, Lobantale, p. xxxi. 



a 1 . Cavity of shell simple. 



b. Apical slit a very long, straight, linear cleft ; shell pol- 

 ished, curved, smooth or with spaced ring-like grooves, 



Fustiaria, p. 127. 

 b 1 . Slit normal or wanting. 



c. Sculptured with extremely oblique, crowded, encir- 

 cling wrinkles, Plagioglypta, p. xxxi. 

 c 1 . Smooth, or with normal growth lines only. 



d. Conspicuously compressed laterally ; apex with 

 a broad slit on the convex side, 



JBathoxiphus, p. 121. 



d l . Subtriangular in section, the concave side wider; 

 apex simple, Gadilina, p. xxxii. 



