ONUSTUS. 207 



OLIVA. Auct. (An olive.) Fam. Convoluta, Lam. Angyosto- 

 mata. Bl. — Descr. Oblong, cylindrical, thick, smooth, shining ; 

 spire very short, with sutures distinct, aperture elongated, notched 

 at both extremities ; outer lip generally thick ; columella thick, 

 obliquely striated, terminated by a tumid, oblique, striated varix ; a 

 raised band passing round the lower part of the body whorl. — 

 Obs. The shells composing this well known genus present a 

 great variety of rich markings and brilliant colours. They are 

 marine and tropical. Fossil species are found sparingly in the 

 London Clay and Calcaire-grossier. The Ancillarise are dis- 

 tinguished from this genus by the sutures of the whorls being 

 covered by enamel. 0. maura, fig. 457. 

 OLIVELLA. Sw. A genus of " Olivinse," Sw. thus described : 

 " Oliviform ; spire (typically) rather produced ; the tip acute ; 

 inner lip not thickened ; outer lip straight ; base of the pillar 

 curved inwards, and marked by two stroug plaits ; upper plaits 

 obsolete or wanting ; aperture effused at the base only ; biplicata, 

 Tank. Cat. 2332. purpurata. Zool. 111. ii. 58. f. 1. mutabilis. 

 Say. eburnea. Zool. 111. ii. 58, f. 2. conoidalis. Lam. No. 57. 

 oryza. Lam. No. 62." 

 OLYGYRA. Say. Mentioned by Ranz as properly belonging to 



Helicina. H. neritella, Auct. 

 OMALAXIS. Desh. Subsequently Bifrontia. Desh. Fig. 354. 

 ONISCIA. Sow. (G. B.) Fam. Purpurifera, Lam. Entomosto- 

 mata, Bl. — Descr. Oblong, sub-ovate, slightly turbinated, can- 

 cellated ; spire short; aperture elongated; terminating anteriorly 

 in a very short, recurved canal ; outer lip thickened, denticu- 

 lated within ; inner lip spread over a portion of the body whorl, 

 granulated.— Obs. The granulated inner lip is the principal 

 character by which this genus is distinguished from Cassidaria. 

 In Oniscia the canal is not so produced. 0. oniscus, fig. 409. 

 ONUSTUS. Humph. A genus proposed by Humphrey and adop- 

 ted by Swainson who describes it thus : " Shell trochiform ; the 

 surface irregular, and often covered with extraneous bodies, ce- 

 mented and incorporated with the calcareous substance of the 



