PLEUROTOMARTA. 41? 



Bibliography, 8rc. — Pleiirotomaria fasciata represents a group rather than a 

 species, since forms which may fairly be classed as varieties are numerous. 

 Indeed, the exact Dundry type is not often met with elsewhere. Deslongchamps 

 (vol. cit., p. 53) expressed a suspicion that his var. platyspira was possibly identical 

 with Trochus fasciatus, Sow., but I have followed d'Orbigny in regarding them as 

 distinct. The following description of PL fasciata is based on Dundry specimens. 

 Description .- 



Height . . . . .65 mm. 



Basal diameter . . . .67 mm. 



Spiral angle ..... 66°. 

 Shell conical, moderately umbilicate. Spire rather obtuse towards the apex, 

 otherwise nearly regular. Whorls (about nine) of moderate and regular convexity, 

 and ornamented throughout by a complete system of rather fine spirals which in 

 the earlier whorls are decussated so as to produce a reticulate pattern ; sutures 

 very distinct, but not deeply impressed. 



The sinus-band occupies the crown of the convexity in each whorl, and is broad 

 and almost median in position, being rather salient in the earlier whorls, flat and 

 strap-like in the later ones ; the ornaments are composed of two or more spirals 

 with the usual cross-hatching, but in the later whorls these are generally worn 

 smooth. The body- whorl is full, subangular at the periphery, and spirally striated 

 both in the flank and base, in addition to growth -lines radiating from the margin 

 of the narrow funnel-shaped umbilicus. Aperture subquadrate to oval. 



Relations and Distribution. — The " Gyrocyclas " and, to a certain extent, the 

 " Gyroplatas " of Deslongchamps are more or less related to PI. fasciata, which 

 may be accepted as a general term where specimens do not admit of any close 

 differentiation. 



This species has a wide distribution throughout the Inferior Oolite in this 

 country. The finest specimens known to me are from Dundry, the matrix being 

 the Iron-shot Oolite, which is probably in the Humpliriesiawus -zone. Both the 

 typical form and the variety platyspira occur rarely in the Sauzei-hed at Oborne. 

 In the Cotteswolds it is found chiefly in the form of casts. I have a specimen 

 from the Murchiso use-zone of Coker. It is by far the most abundant species of 

 Pleurotomaria in the Northampton Sand at Duston, where individual specimens 

 are nearly as fine as at Dundry. Small specimens of this species, resembling 

 PL Niortensis, d'Orb., are not uncommon in the " Base-bed ' at Lincoln 

 (Mnrchisonse-zcme) ; subreticulate ornament is very conspicuous in the apical 

 whorls of some of these. 



