994 DR. J. E. GRAY ON CATILLUF. [DeC. 12, 



tions of form and striu'ture 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 scien- 

 tific works. It is the less excusable, as the opercula could in this 

 and in very many cases have been added with scarcely any appre- 

 ciable 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 opeiculated 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 Na- 

 vicella into three subgenera, according to the position of the nucleus 

 of the spire : — I. Apex of sjiire straight, on the edge of shell. 



II. Septariu: Apex of spire submarginal, entire, as N. cookii. 



III. Eluna : Apex slightly elevated above the edge, and recurved 

 laterallv, as N. lapeyi'ousii. These characters appear to me of very 

 slight importance ; and the form of tlie apex of the spire varies in 

 specimens from the same locality, which I should be inclined to re- 

 gard 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 sub- 

 genera are adopted by Chenu in his 'Manual' (p. 338). 



Adams figures the animal of Calillus lineatus (t. 42. f. 4) and the 

 shell and operculum of Catillus j)orcellaHus (t. 42. f. 4 a, b, c). 



The family Neritinid^e may be thus divided into two tribes: — 



I. Nkritinina. Aperture of shell moderate ; spire conical ; oper- 



culum the size of the aperture of the shell, horny, with a shelly 

 plate nearly of the same size as the horny operculum. 



II. Catili^ina. Aperture of the shell very large, ovate; spire ru- 



dimentary ; operculum small, horny, with a shelly plate pro- 

 duced 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 ])roduced beyond the 

 end of the foot ; the free end is generally bifid, with a produced 



