Zoological Illustrations Series II/Plate 40
OLIVA. Pl. 1.
1. volutella 2. striata
OLIVA volutella.
Class Mollusca. Order Zoophaga. Lam.
Genus Oliva. Lam. Sys. 7, p. 416.
Specific Character.
Shell ovate-conic, mitriform; chestnut, the body whorl bluish; spire lengthened, acute; pillar with 6-7, slender, regular plaits.
Oliva volutella. O. testâ ovato-conicâ, subcæruleâ, ad spiram basimque luteo fuscatâ; spirâ valdè productâ, acutâ. Lam. Sys. 7, 1, p. 432.
We are indebted to the Rev. Mr. Bulwar for our specimens of this elegant and highly interesting species, received by him from the American Isthmus. It varies much in the intensity of its colour, as may be seen from the above description, given by Lamarck, of examples procured by Baron Humbolt on the shores of Mexico.
There are reasons for believing the genus Voluta, as defined by Lamarck, to be typical of a natural group, comprising the genera Mitra, Oliva, Ancillaria and Marginella. The obvious affinity between Voluta and Mitra need not be dwelt upon. The connexion of Mitra with Oliva, we noticed in describing Mitra olivæformis, at pl. 48 of our first series. This affinity appears further strengthened by a shell in the late Tankerville Collection, especially recorded by Mr. G. Sowerby, as "remarkable for its resemblance to an Olive." We regret not having seen this shell, but such an authority is fully sufficient. The group of Mitres, by which this transision appears effected, is that mentioned under the head of M. bicolor (Mitra, Pl. 1.) of this series. This little group has three relations: one to the smooth Mitres (as M. pertusa), another to Conohelix, and a third to the Olives: the species are all remarkable for the polished smoothness of their surface, for the total absence of the inner lip, and for the lower plaits of the pillar extending much beyond the aperture: these plaits, moreover, are generally five or six, and all very slender. The conchologist will perceive that nearly the only character wanting, to render this description applicable to an Olive, is the channelled groove round the suture. This character is added to the shell before us, which thus presents the first type of form in the present genus. Proceeding to the more perfect Olives, we observe the spire becoming gradually shorter; the vitrious deposition on the inner lip increased in thickness; the upper plaits on the pillar numerous, and those at the base thicker and more external (M. fulmineus, Lam. elegans, En. Meth. 362, 3.), the ventricose shape of some (O. undata, inflata, Lam.), the spire often concealed by a vitrious covering, and even the colours of others (M. tessellata, guttata) remind us of the typical Volutes. It is very remarkable, that in most of the cylindrical Olives, the apex is more or less, papillary: the last complete terminal volution of the spire being thick, inflated, and distorted: (see particularly O. reticularis, En. Meth. 36, f. 1; fusiformis, Ib. 367, 1; guttata, Ib. 368, f. 2.) Others are seen of a more slender form, as O. subulata, L. acuminata, E. M. 368, 3, where the pillar is similarly plaited. But in several small species allied to O. conoidalis, another modification of form has evidently commenced: the spire is regularly acute; the upper plaits disappear; and those which remain, are only at the thickened base of the pillar: the aperture, no longer narrow, becomes wide at the base, where the pillar takes an oblique direction inward: we are thus led to the form of O. hiatula, E. M. pl. 368, f. 5, where the characters of Ancillaria become apparent in the effuseness of the mouth, the double belt at the base, and the imperfect groove on the outer side of the pillar. Whether we look to this shell, or to the descriptions given by Lamarck of the fossil species plicaria, canalifera, and laumontiana, or finally, to the Ancillaria glandiformis, Sow. no doubt can remain of the genus Oliva being here blended with the Ancillariæ. There is, however, another form among the Olivæ, which deserves mention, as it cannot well be associated with either of the preceding: this we have described below. Another genus that may be thought connected with Oliva, is Terebellum: a group very ably illustrated by Mr. G. Sowerby, in his "Genera of Shells." To that work we must refer the reader for those reasons which shew its more immediate relation to Ovula. It is, in short, one of those forms too hazardous to class without a complete knowledge of the animal.
OLIVA striata.
Shell small, cylindrical, sub-effuse, distinctly marked by regular longitudinal striæ; upper part of the body whorl with one, lower part with four, belts; spire short, channelled, sub-papillary, naked; base of the pillar striated and externally detached by a groove.
Ancillaria canalifera?? Lam. Syst. 7, 415.
We do not find this fossil clearly described. Our specimens appear to be from the London clay. Its form seems analogous to Conohelix among the Mitres. The line adjoining the figures denotes the natural length of the shells.