130 MALACOZOA. GASTEROPODA. PECTINIBRANCHIATA. 



The occurrence of dead terrestrial and fluviatile shells of 

 other districts on our shores may be accounted for in various 

 ways. For example, Nerita fluviatilis, and Bithinia tentaculata, 

 may have come to Aberdeen in mud or sand, from England, 

 used as ballast. 



Nerita fluviatilis. Linn. Syst. Nat. 1253. — Nerita fluviatilis. 

 Muller, Verm. Terr. et. Fluv. ii. 194. — Nerita fluviatilis. Drap. 

 Moll. Terr. et. Fluv. 31. PI. 1. f. 1, 2, 3, 4.— Nerita fluviatilis. 

 Penn. Brit. Zool. iv. 141. PI. 88. f. 142 ; Ed. n. iv. 345. PI. 90 

 f. 2. — Nerita fluviatilis. Mont. Test. Brit. 470. — Neritina fluviatilis. 

 Flem. Brit. Aniin. 321. — Neritina fluviatilis. Lamk. Syst. vi. ii. 

 188; Ed. n. viii. 576. — Neritina fluviatilis. Gray's Turton. 



Family III. — Turbinina. 



Animal with the body elongated, spiral, and covered 

 with a shell ; head with a frontal veil ; two large conical 

 tentacula; eyes on prominences at their base externally; 

 mouth toothless, but with a spiral lingual filament ; 

 respiratory cavity containing two unequal pectiniform 

 branchiae, its opening without tube ; organs of genera- 

 tion in the female opening into the branchial cavity, in 

 the male near the base of the right tentaculum. A horny 

 or calcareous operculum. 



Shell spiral, globose, ovate, conical, or turrite, with 

 the aperture round, ovate, pyrate, semicircular, or ob- 

 long, rounded anteriorly, without canal or notch. 



All the species are marine, although some inhabit 

 brackish water, and are phytophagous, living chiefly on 

 Algae. 



This family, of which the organization is similar 

 throughout, has been variously subdivided by authors 

 according to the form of the shell, and especially of its 

 aperture into a number of distinct families. For example, 

 Blainville arranges his order Asiphonobranches into Fam. 

 1 . Goniostomes, or angular-mouthed shells ; Fam. 2. 

 Cricostomes, or round r mouthed ; Fam. 3. Ellipostomes, 

 or elliptical-mouthed. These three families however 

 present no essential differences in the structure of the 

 animal, and are all here included under the general name 

 of Turbinina, Used even sectionally such an arrange- 



