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 Ancillariae 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 " Olivinee," 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 strong 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 : fi Shell trochiform ; the 

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

 mented and incorporated with the calcareous substance of the 



