RISSOA. 95 



and Rack. Trans. Linn. Soc. vol. viii. p. 162. — Turt. 

 Conch. Diction, p. 201. — Dillw. Recent Shells, vol. ii. 

 p. 837.— Wood. Index Testae, pi. 30, f. 50. 

 C«H^w7a sfrMto, Fleming, Brit. Animals, p. 307. — Brit. Marine Conch, p. 178. 



f. 99 (not well). 

 Bissoa minuiissima, Michaud, Nouv. Esp. de Riss. p. 20, fig. 27, 20. — Desk. 

 Anim. s. Vert. vol. viii. p. 480. — Potiez and Mich. Gal. 

 Doiiai, Moll. vol. i. p. 274 — Recluz, Revue Zool. Cuvier. 

 1843, p. 6. 

 „ communis, Forbes, Malacol. Monensis, p. 17. 



„ striata, (not Quoj', nor Desh.) Johnston, Berwick. Club, \<A. i. p. 271 

 (with animal). — Macgilliv. Moll. Aberdeen, p. 152. 

 Cingula semicostata, Fleming, Brit. Animals, p. 307. — Brit. Marine Conch, 

 p. 177. 



Rissoa gracilis, M.ACGiLh. Moll. Aberd. p. 152, from which Cingula gracilis, 

 Brit. Mar. Conch, p. 262 ; copied also. Brown, Illust. Conch. 

 G. B. p. 129. 

 Odostomia semicostata, Macgill. Moll. Aberd. p. 155. 



„ Mariona, Macgill. Moll. Aberd. p. 156 (teste Jeffr. from type) ; 

 copied, Brit. Mar. Conch, p. 260, and Brown, 111. Conch. 

 G. B. p. 130. 

 Rissoa decussata. Brown, Illust, Conch. G. B. 

 Piiramis candidus and discoj-s. Brown, Illust. Conch. G. B. p. 14, pi, 9, f. 31, 



32 

 Rissoa semicostata. Brown, 111. Conch. G. B. p. 11, pi. 9, f. 1, 2. 



The very different look of the adult and immature 

 individuals of this most abundant shell, induced Montagu 

 to regard the latter as a distinct species, which he pub- 

 lished under the name of T. semicostatus. The 0. 3Iarionce 

 of Macgillivray is the same in a rather more advanced 

 stage, the relative proportions of the spire and mouth 

 varying of course with age, and especially in this Rissoa, 

 whose penult turn is nearly as large as the final volu- 

 tion. The R. gracilis of the same author was also de- 

 rived, observes Mr. Jeffreys, after an inspection of the 

 type, from a slender worn specimen of this species. 



The form of this shell, although varying greatly as to 

 elongation, is always remarkably narrow, being cylindra- 

 ceous below, and convexly tapering above. Characteristic 



