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Show 994 DR. J. E. GRAY ON CATILLUS. [Dec. 12, tions of form and structure that they at once distinguish the species, and serve to divide them into most natural groups. The non-attention to such particulars greatly diminishes the value of the ' Conchologica Iconica,' and removes it from the category of scientific works. It is fme less excusable, as the opercula could in this and in very many cases have been added with scarcely any appreciable additional trouble. The fact is that I suppose Mr. Reeve intended it for a mere shell-dealer's catalogue, and not a scientific conchological and malacological work. The species that are in a perfect condition (that is to say, that are accompanied by their opercula) in Mr. Cuming's collection may be arranged as in this essay. An operculated shell without its operculum is in a very imperfect condition, as the operculum is most important in the organization and economy of the animal, and it often affords most important characters for the distinction of the species and the determination of the genus to which the animal and shell is to be referred ; so that an operculated shell not accompanied by its operculum must be regarded as wanting one of its most important organs. Messrs. H. & A. Adams, in their ' Genera' (p. 386), divide Navicella into three subgenera, according to the position of the nucleus of the spire:-I. x\pex of spire straight, on the edge of shell. II. Seqjtaria: Apex of spire submarginal, entire, as N. cookii. III. Elana: Apex slightly elevated above the edge, and recurved laterally, as N. lapeyrousii. These characters appear to me of very slight importance ; and the form of the apex of the spire varies in specimens from the same locality, which I should be inclined to regard as the same species. The operculum of the genus is described by them as quadrangular. This would exclude Navicella lineata, which these authors referred to the second subgenus. These subgenera are adopted by Chenu in his 'Manual' (p. 3'*>8). Adams figures the animal of Catillus lineatus (t. 42. f. 4) and the shell and operculum of Catillus porcellanus (t. 42. f. 4 a, b, c). The family NERITINID.E may be thus divided into two tribes: - I. NERITININA. Aperture of shell moderate; spire conical; operculum the size of the aperture of the shell, horny, with a shelly plate nearly of the same size as the horny operculum. II. CATILLINA. Aperture of the shell very large, ovate; spire rudimentary ; operculum small, horny, with a shelly plate produced beyond the edge of the horny operculum, which with the operculum is much smaller than the aperture of the shell. The operculum of Catillina, as in other shells which are said to have a shelly operculum, consists of two parts:-1, the horny plate, which is affixed to the hinder part of the foot of the animal; 2, the shelly part, which is on the outer side of the horny plate, or true operculum, and which, as it increases in size, is produced beyond the end of the tout; the free end is generally bifid, with a produced |