;82 



DIVISIONS OF THE GASTROPODA. 



Fig. 681. — Scalaria 

 Grcenlandica. Post- 

 Pliocene and Recent. 



appear in the Trias, and a number of Jurassic and Cretaceous 

 species are known ; while the genus is largely represented in the 

 Tertiary rocks and at the present day. In the Jurassic genus 

 Exelissa, the essential characters agree with 

 those of Scalaria, but the shell is of small 

 size, the aperture is constricted, and the last 

 whorl may be partially disjunct. The Triassic 

 genus Cochlearia differs from Scalaria in having 

 the whorls keeled and spirally striated, the trans- 

 verse ribs being feebly or not at all developed, 

 while the mouth is thickened and trumpet-shaped ; 

 and the Devonian genus Scoliostoma resembles 

 Cochlearia except that the last volution is bent 

 upwards and the surface is cancellated. Lastly, 

 we may place here the ancient genera Callone?na 

 and Holopella, which agree with the typical mem- 

 bers of this family in having a conical shell with 

 an aperture surrounded by a continuous peristome. 

 Callonema ranges from the Ordovician to the De- 

 vonian, and has the surface ornamented by re- 

 mote, transverse, lamellar ribs ; while in Holopella the ribs are 

 reduced to mere striae, or are obsolete. 



Family 22. Ianthinid^e. — The type of this family is the singular 

 pelagic genus lanthina, in which the shell is thin and turbinated, 

 with an oval or subquadrate aperture, the outer lip being thin, and 

 the columella slightly twisted. On the ground of similarity as re- 

 gards the characters of the radula, lanthina is placed by Fischer in 

 the vicinity of the Scala?'iidce. The only known fossil forms are 

 from the Pliocene deposits of Italy. 



Family 23. TuRRiTELLiDiE. — In this family the shell is spiral, 

 many-whorled, turreted, the surface spirally ribbed or striated. The 

 aperture is round or subquadrate, entire, but sometimes slightly 

 notched in front. The outer lip is thin, and the operculum is 

 horny. All the members of this family are marine, and, if Vermetus 

 and Ccecum be excluded, the principal representative of the group is 

 Turritella itself (fig. 682), the characters of which are therefore 

 those given above. Many living Turritellce are known, and a still 

 larger number of fossil forms have been described, the earliest un- 

 questionable representatives of the genus occurring in the Jurassic, 

 or perhaps in the Triassic deposits. Most of the fossil species, how- 

 ever, are found in the Tertiary rocks. 



Family 24. Vermetid^e. — In this family the shell (fig. 683) is 

 tubular, more or less spiral at first, but with its last turns disjunct. 

 The aperture is round, and is entire or furnished with a lateral slit. 

 A horny circular operculum is usually present. All the members of 



