998 DR. J. E. GRAY ON CATILLUS. [DeC. 12, 



notch on its left margin (Figs. 3 c, 3 b). Shell elongate, mouth 

 elongate, narrow. 



Fig. 3 a. Fig. 3 b. 



Fig. 3 a. Outer surlace. Fig. 3 h. Inner surface. 



The operculum has a very thin brittle shelly plate, which is sepa- 

 rated from the horny (or true) operculum by a distinct straight hue 

 running from the centre of the right side of the operculum to the 

 outer side rather above the base. The outer surface of the shelly 

 plate is slightly convex, smooth, very finely concentrically striated, 

 w^ith a diverging rib from the nucleus to the upper edge, and with 

 another more distinct linear rib on the right margin, which ends in 

 a slender spine ; the space between the diverging ribs is depressed 

 and striated, like the rest of the surface. According to the figure 

 of the animal in Adams's 'Genera' (p. 386, t. 42. f. 4), the end of 

 the operculum is produced beyond the end of the foot. 



Stenopoma, n. g. 

 Stenopoma lineatum. (Operculum, Fig. 3.) 



Navicella Uneata, Lamk. E. M. t. 456. f. 2 ; Reeve, t. 8. f. 31. 



N. tessellata, Lamk. ; Reeve, t. 6. f. 27. 



Septaria navicula, Ferussac, Bull. 



Hab. Philippine Islands. 



Lamarck divided these shells into two species, according to the 

 coloration, calling one lineata and the other tessellata ; but the two 

 systems of coloration gradually pass into each other in the different 

 specimens ; and some specimens present the two kinds of coloration 

 each well marked : for example, one was C lineata when young, 

 and became C. tessellata afterwards ; so that the upper part of the 

 shell is of one species, and the margin of the other. 



The specimens from the same locality present considerable varia- 

 tion in the general form of the shell, some being much narrower and 

 more convex than others, and also in the form, position, and extent 

 of the hinder lip. 



These shells have a general external resemblance to the com- 

 pressed Limpets {PatellcB), that live in the cavities which they eat 

 out at the base of the roots of Alg^B, or on the cylindrical stems of 

 those plants. They do not appear to derive their form, like those 

 shells, from adapting themselves to the form of the body to which 

 they happen to be attached ; and, indeed, Mr. Cuming states that 

 the specimen which he procured in the Philippines lived on stones, 

 like the Catilli. Though they are compressed and oblong, the base 

 of the cavity is flat and even, as much so as the circumference of 



