1890.] MARINE MOLLUSCA OF ST. HELENA. 281 



peristoma fere continuutn, margine columellari paulo prominente, 

 superne toi-to. 

 Longit. 2i millim., diam. g. 



This minute species is very like A. nitidissima of Montagu, but 

 has decidedly less convex whorls, the aperture is broader, and the 

 columellar twist different. The heterostrophe apical coil is also very 

 similar in both forms. 



AcLis DiDYMA. (Plate XXIII. fig. 30.) 



Testa minuta, turrita, albida, imperforata ; anfractus 6, supremus 

 liEvis, convexus, obtusus, cceteri superne declives, subexcavati, 

 dein obtuse angulati, inferne planiusculi, longitudinaliter striati, 

 ad angulum subplicati ; apertura parva, ovata, longit. totius 

 \ adcequans ; columella levissime rejlexa, superne subtorla. 

 Longit. 2j millim., diam. |. 



Owing to the large size of the nuclear whorl, this species has very 

 gently converging outlines. The columella does not unite above 

 with the outer lip, but appears to be slightly spirally intorted. 



Solarium placentale, Hinds, var. 



Hah. Bay of Magdalena, California. Off Barbados in deep 

 water (Dall for S. jjeracutum). 



Three specimens in excellent condition, one alive with the oper- 

 culum, were dredged by Capt. Turton. This is anotlier insrance of 

 remarkable distribution in this genus. After a careful study and 

 comparison of these examples with the types of S. placentale, and 

 Mr. Ball's description and figures of S. peracutum ^ although slight 

 differences are noticeable, I can but regard all of them as forms of 

 one and the same species. The St. Helena specimens are a little 

 paler in colour than the type ; the periphery is perhaps very slightly 

 more acute, as is the case with S. peracutum ; the crenulations bor- 

 dering the umbilicus finer, and the spiral sculpture, more especially 

 on the upper surfaces, is rather more inclined to be granular. 



The operculum consists of sis whorls, which rapidly increase from 

 a central nucleus, and, on the external surface, have the outer margin 

 elevated, forming a sutural keel and thus giving them a concave 

 aspect. The inner surface is glossy and furnished with a strong 

 whitish central elevated process, from which a conspicuous curved 

 ridge arises, forming rather more than a semicircle. 



The figure of S. placentale in the ' Conchologia Iconica ' is a mere 

 caricature, being both out of drawing and exaggerated in colour and 

 sculpture. The figures in the ' Voyage of the Sulphur ' (pi. xiv. 

 figs. 5, 6) are good and of the natural size. 



The acuteness of the peripherial keel is variable, for, in a second 

 specimen of the typical form, received by the British Museum from 

 Sir E. Belcher, it is sharper and flatter above. 



Solarium ordinarium. (Plate XXI. figs. 17-17 5.) 



Testa orbiculo-conoidea, depressa, mediocriter umbilicata, albida 



1 Bull. Mus. Comp. Zool. Harvard, vol. xviii. p. 275, pi. xxxiii. figs. 2, 5. 



