M. Deshayes' Monograph of DentaUum. 181 



^^ar. a. Testa minor e, costis minus eminentibus. 



Var. b. Testa minore sex costatd, striis binis vel tribus inter castas. 



Foss. Placentise et in Austria. 



Very large, slender, considerably curved, constantly exhibiting twelve ' 

 ribs, six of which are more prominent, and are prolonged to the apex, 

 while the other six do not reach so far, and usually continue smaller. In 

 the older individuals the ribs become striated, and disappear insensibly 

 towards the opening. A'^ery fine and irregular transverse strise indicate 

 the growth ; the apex is constantly hexagmial ; the opening is oblique, 

 with cutting and simple margins. 



Length from 4 to 4^ inches. Diameter of the opening scarcely five 

 lines. Length of the first variety 2 inches ; of the second 1 inch 6 lines. 

 In the latter the six intermediate ribs are each replaced by two or three 

 strongly marked striae. 



4. D. APRiNUM, Linn. D. testd virescente, tereti, subsulcatd, decern cos- 

 ■ tatd, striis transversis nullis ; apice acuminato. 



Var. a. Testd duodecim rcgidariter costatd, albidd. 



Var. b. Testd undecim costatd : stria unicd inter castas. 



D. striatulum. Gmel. p. 3738, n. 13. 



Habitat in mari Indico, necnon in Mediterraneo. 



Resembles D. arcuatum, but is slenderer and more curved, its green 

 colour is less deep, and extends equally to tlTe apex, which is less acu- 

 minated. Ribs varying in number from ten to twelve, symmetrical, nar- 

 row, placed at regular distances, and equally prominent on the concave 

 and convex surfaces, extending from the apex to the base. Transverse 

 strise, indicative of the growth, few and irregular, generally accompanied 

 by zones of a deeper green ; the remaining space between the ribs being 

 entirely smooth. 



Length 2 inches 7 hues. Diameter of the base 2^ Hnes. 



5. D. OCTOGONUM, Lam. D. testd albidd lacted, subarcuatd, oclogo- 

 nd ; costis octonis ; duabus tribusve striis inter castas. 

 Var. a. Testd septem costatd. 

 Habitat in mari Sinensi. 



