PROSOBRANCHIATA. 78 1 



ous peristome. The species of Rissoa are inhabitants of the sea, 

 and the range of the genus in time is from the Jurassic to the 

 present day. Forms of sub-generic value, or allied types, are Ris- 

 soina (Jurassic to Recent), Keilostoma (Chalk to 

 Oligocene), Diastema (Eocene), and Pterostoma 

 (Eocene). 



Hydrobia is nearly related to Rissoa, but the shell 

 is usually thin and smooth. The species of this 

 genus are mostly inhabitants of brackish or fresh 

 waters, but a few live in the sea. The earliest forms 

 of Hydrobia are recorded from the Jurassic rocks, 

 but the genus is mainly Tertiary and Recent. 

 The genus Assiminea differs from Hydrobia almost supi'adst^ta, 1 ^- 

 wholly as regards the animal, but forms are stated pJJcene Slx (After 

 to occur in rocks as old as the Eocene. Searies Wood.) 



Family 20. Littorinid^e. — In this family the 

 shell is spiral, top-shaped, not nacreous internally, with a rounded 

 and entire aperture and a simple thin outer lip. The operculum 

 is horny and paucispiral. The members of this family are wholly 

 marine ; and their shells are very similar to those of the Trochidcc 

 and Turbinidtz, except that there is no nacreous layer developed. 



In the genus Littorina are the true Periwinkles, distinguished by 

 their thick, generally top-shaped and pointed shells, of few whorls, 

 and with an imperforate columella. The undoubted fossil species 

 range from the Jurassic to the present day. In 

 the Tertiary and Recent genus Lacuna, the shell 

 resembles Littorina in most respects, but it is 

 thin, and the columella is flattened and is bor- 

 dered by an umbilical fissure. We may also 

 include here the Silurian and Devonian genus 

 Holopea (fig. 680), in which the shell is spiral, 

 and either conical, or, more usually, naticoid, Fig. 6Zo. — Hoiopea 

 with a short spire and a large body-whorl. The gggjT**' < BiIlin * s -> 

 surface is smooth or marked with faint trans- 

 verse striae ; the outer lip is thin ; the aperture is entire ; and there 

 may or may not be a narrow umbilicus. 



Family 21. Scalariid^e. — In this family the shell is spiral and 

 turreted, many - whorled, the volutions marked with longitudinal 

 ribs ; and the aperture is round and furnished with a continuous 

 peristome. The shell is perforated, but the umbilicus is often closed 

 or concealed ; and the operculum is horny and spiral. 



In Scalaria itself are included the " Wentle-traps," in which the 

 shell is elongated and many-whorled (fig. 681), and the volutions 

 are adorned with transverse ribs, while the peristome is continu- 

 ous round the circular aperture. The earliest forms of this genus 



