PROSOBRANCIIIATA. 207 



tertiary deposits. In the Western hemisphere, which presents a large proportion of 

 the living species, the genus does not appear to have had so large a development ; 

 twenty-three species only from the tertiary deposits in the United States have been 

 described by Conrad and Lea, and three species from the newer tertiaries of Chili have 

 been described by Sowerby. The genus is largely represented in the English eocene 

 fauna, but as yet comparatively few species have been described or identified. 



The shells of the Pleurotoma appear to have been peculiarly subject to modification 

 by external conditions, and, as De Blainville has remarked, it is " apparently with 

 them as with the Cerithia, the Ammonites, and other genera which contain many 

 species; each locality presents different forms." 



Section I. Shells fusiform. 

 A. Sinus in the posterior margin of the whorl, 

 a. Canal elongated. 



No. 133. Pleurotoma stena, F. E. Edwards. Tab. XXV, fig. 4 a, b. 



P. testa elongato-fusiformi, angustd, sub-turritd, fasciis et Jilis spiralibus, lineis incre- 

 menti decussatis, omnino tectd ; spird productd, apice acuminato : anfractibus convexius- 

 culis, angulatis, obtuse carinatis, postice sub-concavis, ad suturam crenulatis ; fills concen- 

 tricis numerosis, inaqualibus ; anfractu ultimo antice gradatim attenuato et in canali longo. 

 recto, exexmti •. aperturd elongato-ovali ; labro vix arcuato ; sinu labrali angusfo, prof undo 

 in margine collocato. 



Shell elongate, fusiform, narrow, ornamented with numerous spiral bands, the 

 spaces between which, as well as the surface between the shoulder and the suture, 

 and sometimes even the spiral bands, are covered with numerous fine, thread- 

 like, unequal, raised lines, decussated, or rather roughened, by the lines of growth ; 

 the spire, which is formed of seven or eight volutions, is much produced ; the whorls 

 are slightly convex, sharply angulated at the shoulder; the posterior margins de- 

 pressed and slightly thickened at the edge, where one or two raised lines, stronger than 

 the others, and crenulated by the lines of growth, run round the suture ; the space 

 between the suture and the shoulder is concave, giving somewhat of a turreted aspect 

 to the spire. The spiral bands are irregular, narrow, flat on the surface, sharp edged ; 

 the posterior band runs round the shoulder, forming a blunt keel, and the space 

 between it and the band immediately in front of it is wider than those between the 

 other bands, and is concave ; the bands, as they approach the anterior part of the 

 shell, become closer, narrower, and less prominent, while, on the other hand, the con- 

 centric lines become stronger, more elevated, and more distant, until the two blend 

 together, and form the round, coarse, raised lines which cover the base of the shell 



