82 ZOOLOGY. 



shell, where it forms a notch ; and the apex of the shell is directed back- 

 wards, whilst in Fissurella it turns forwards. 



Fam. 2. CalyptrcEidce. The shell of Calyptrtea is patelliform or trochi- 

 form, irregular, somewhat conical, with the apex rather posterior. Towards 

 the apex of the inside there is a plate which is sometimes funnel-shaped, 

 and sometimes like the vertical half of a funnel. In other species it forms 

 a spiral, which approximates the slieil to that of the spirivalve genera. The 

 animal is not spiral, the two tentacles are large and triangular, having the 

 eyes upon an enlargement of the external side ; the branchial cavity is 

 large and oblique,- and the branchise are pectinated, filamentous, and 

 exsertile. Calyjjtrcea sinensis {jpl. 75, jig. 78). 



Orepidula has an ovate or oblong shell, generally convex above, the 

 internal cavity divided some distance by a shelly diaphragm which 

 represents the plate in Calyj)trfea, and above which the viscera are placed. 

 The animals of these two genera are alike, and the shells of some species 

 present intermediate characters, so that it is difficult to tell to which of the 

 two genera they should be referred. The Crepidulffi are sedentary, seating 

 themselves upon stones, or upon each other, and adapting the margin of 

 their shells to the irregularities of the ol)ject upon which they place 

 themselves. 



Obdek 9. NEMATOBKANcniA (or CirrJiohixmcMa). The genus Denfalium 

 {jjjl. 75,^^5. 73-75) was for a long time believed to belong to the Annellida, 

 until dissection proved it to be a mollusc. The shell is a long, slender, and 

 .slightly curved cone, open at both ends, and sometimes ribbed or striate 

 externally, as in D. elej)hantinnin [fig. 73). The convex part of the shell 

 corresponds to the back of the mollusc. The branchige are composed of 

 numerous extensible filaments forming a bundle upon each side of the neck, 

 and it is probable that the branchiae can be used to convey food to the 

 mouth. The vent is at the posterior or smaller end of the shell, the lips are 

 scolloped into a number of palpiform projections, and the ordinary tentacles 

 and eyes are absent. The animal lives vertically in the sand with tlie head 

 downwards. There are some shells which closely resemble those of 

 Dentalium, and which were classed with them until it was ascertained that 

 their animal is an annellid. They form the genus Ditruj^a. 



Okdek 10. Ctenobranchia (also named Pc'Ctinibranchia). This is an 

 -extensive order of dioicous spirivalve mollusca, in which the branchiae are 

 pectinated, one or two in number, and placed in a large cavity above the 

 neck ; the tongue is armed with numerous teeth, the tentacles and eyes are 

 usually two in number, and the aperture is usually closed by an operculum. 

 Most of the families are marine, but some inhabit fresh water. The families 

 xire as follows: 1, Melaniidos; 2, Cerithiidse; 3, Vermetidse; 4, Trochidse; 

 5, Pyramidellidffi ; 6, Buccinidse ; 7, Purpuridae ; 8, Strombidse ; 9, Conid^ ;' 

 10, Cyprseidse ; 11, Yolutidse ; 12, Sigaretidsa. 



Fain. 1. IfelaniidcB. Menke's corresponding family named Turhinea is 



inapplicable, because in its various modifications it is used for the family to 



which Turbo belongs, by Lamarck and Deshayes, and by Blainville for a 



family of Polythalamia. Some of the genera of this family inhabit the sea, 



286 



