PLEUROTOMARIA. 399 



335. Plecjrotomaria elongata, Sowerby, 1818. (= PL mutabilis, Deslongchamps, 



pars.) Plate XXXIII, figs. 4, 5, 6, and 7; 

 Plate XXXIV, fig. 8. 



ISIS. Tsocnus elongatus, Sowerby. Min. Conch., pi. cxciii, figs. 2 and 3. 



1854. Pleubotomabia elongata, Sowerby. Morris, Cat., p. 271. 



1873. — — — Tawney, Dundry Gasteropoda, p. 37 



(29). 



Syn. — mutabilis, Deslongchamps, vars. elongata, mutica, and 



ambigua. Mem. Soc. Linn. Norm., vol. 

 viii (" Les Pleurotomaires "), p. 108, &c, 

 pi. x, figs. 14 and 15; pi. xi, fig. 1. 



Bibliography, Sfc. — Since this is a group rather than a species, in the stricter 

 acceptation of the term, it would be useless to attempt a full synonymy. Tawney 

 included the following, with more or less doubt, under this heading, viz. PI. 

 abbreviata, Sowerby, PI conoidea, Deshayes, and PL Ebrayana, d'Orbigny. The 

 above, together with other named forms, are doubtless connected, and may for the 

 most part be regarded as varieties. Nevertheless Deslongchamps, who attached, 

 little value to the spiral angle as a means for determining species in Pleurotomaria 

 made his PL mutabilis a little too comprehensive. Since Sowerby originally 

 recognised two species in this group, viz. PL elongata and PL abbreviata, I have 

 concluded to follow his example. 



It will be observed in the sequel that PL elongata, as thus limited, covers a 

 considerable variety of forms according to the horizon, the lower beds usually pre- 

 senting the narrower forms, although in the Parkins oni-zone at Burton Bradstock 

 and elsewhere are a number of small specimens, many of which may be allotted to 

 PL elongata in a general sense. 



General Description. — Shell conical-elongate, not umbilicated. Spire regular 

 or slightly concave, the spiral angle ranging from about 40° to somewhat less 

 than 50°. Whorls about twelve, excavated, and terminated by a basal rim which 

 projects over each succeeding whorl and constitutes an important feature. This 

 rim is crenulated in some cases, and subcrenulate to smooth in others, presenting 

 considerable variety in its sculpture. The intercarinal spaces have rich spiral 

 ornament, slightly reticulate in the earlier whorls, subgranulate in the later ones. 



The sinus-band is very anterior, rather narrow, and but slightly prominent, 

 usually presenting one median spiral cross-hatched by growth lines, but in some 

 cases the ornamentation is more complex. The body-whorl is angular at the 

 periphery, with a flat base which is not as a rule excavated ; base spirally grooved ; 



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