780 DIVISIONS OF THE GASTROPODA. 



body -whorl and wide aperture. The species of this genus are 

 Tertiary and Recent. 



Family 18. Paludinid^:. — In this family the shell is conical or 

 globular, with a thick epidermis, and with convex whorls, the aper- 

 ture being entire and sometimes furnished with a continuous lip 

 The operculum may be horny or shelly. The members of this 

 family are essentially fresh-water forms, though some are found in 

 brackish waters, and Ampullaria is known in some cases to inhabit 

 salt water. As a matter of course, therefore, they are chiefly, if not 

 exclusively, found as fossils in deposits which are believed to be 

 fluviatile, lacustrine, or estuarine in origin. 



The genus Vivipara (Paludind) comprises forms with a conical 

 shell, a generally smooth surface, and a horny operculum, an umbili- 

 cus being sometimes present, sometimes absent (fig. 678, a). The 



oldest forms of Vivipara, in the wide 

 ^^ sense, are found in the Lower Cre- 

 ||u~^ taceous rocks (Wealden Beds), and 

 T^© a large number of Tertiary and Re- 

 cent species are known. Bithynia 

 #2\ resembles Vivipara, but the oper- 

 f ^C#\ culum is partially calcined ; and its 

 B ^gP' range in time is the same, though 

 the known fossil forms are few in 



Fig. 678. — a, Paludina lenta, Pliocene ; ■, t ir i j. tc c o \ 



B, Valvata piscinalis, viewed from in number. In ValVata (hg. 676, B) 



f w o n od a ) nd from below ' (After Searles the sheU ma y be t0 P- sha P ed 0r dis " 



coidal ; an umbilicus is present, and 

 the peristome is entire. The earliest undoubted forms of this genus 

 appear in the Upper Jurassic rocks (Purbeck Beds), and a small 

 number of Tertiary and Recent forms are known. Lastly, in Am- 

 pullai-ia (fig. 639) the shell is globular, with a short spire and a 

 ventricose body-whorl, and with or without an umbilicus. The 

 operculum may be horny or calcareous. The geological range of 

 Ampullaria is a matter of uncertainty, from the difficulty, or impos- 

 sibility, of separating the shells of this genus from those of Natica 

 and its allies. Forms from rocks as old as the Lias have been re- 

 ferred to Ampullaria, but it is doubtful if any representatives of the 

 genus are known to occur in the fossil condition. 



Family 19. Rissoid^:. — In this family the shell is spiral, usually 

 with an elevated spire, the aperture being entire and rounded, and 

 the operculum horny and spiral. The members of this family are 

 partly marine, partly inhabitants of brackish waters or marshes ; and 

 the two principal genera comprised in it are Rissoa and Hydrobia. 



In the genus Rissoa (fig. 679) are included small, thick shells, 

 generally more or less ribbed or striated, pointed in shape, many- 

 whorled, and with a small round aperture surrounded by a continu- 



