76 MOLLUSCA FROM THE GREAT OOLITE. 



Phasianella tumidula. Plate XI, figs. 25, 25a. 



P. Testa turbinatd, elongatd; spird acuta; anfractibus convexis (8), suturis depressis; 

 anfractu ultimo globoso; aperturd magna ovato-rotundatd. 



Shell turbinated, elongated; spire acute; whorls (8) convex, the sutures deeply 

 depressed ; the last whorl globose ; the aperture large, ovately rounded. 



This species has an elevated, acute spire, and convex whorls, and is remarkable for the 

 sudden increase of the last two volutions, which are very ventricose. Neither of our 

 specimens are quite perfect about the outer lip ; but the distinctive character of the species 

 is sufficiently evident. Axis 19 lines, transverse diameter 11 lines. 



Locality. It occurs rarely in the planking at Minchinhampton Common. 



Family — Pleurotomarid^. 

 Pleurotomaria, DeJ ranee. 1825. 



Scissurella, D'Orbigny. 1823. 



Shell turbinated or conical ; aperture subquadrate, the angles rounded ; outer lip thin 

 and sharp, having a fissure or deep notch in the middle part, or near to the suture ; an 

 encircling band or rib round each whorl follows the fissure. 



The Pleurotomaria are rare in the Minchinhampton beds, and the larger specimens are 

 usually broken. It will be observed, in the following descriptions, how very few examples 

 of each species have been obtained, so that we are almost enabled to give their number 

 with exactness. Placed amidst such a multitude and variety of molluscous relics, in spots 

 teeming with life, it is not easy to account for their rarity and imperfect condition. 

 Inferring that they were usually gregareous, we are led to suspect that the littoral condition 

 of these shelly beds was not suited to their propagation, and that the larger imperfect 

 specimens were denizens of greater depths, the shells occasionally being stranded among 

 the more littoral Mollusks. As a remarkable instance of the recurrence of similar phe- 

 nomena at a very distant locality, we would direct attention to the elaborate and valuable 

 Memoir of M. Deslongchamps, 1 on the Pleurotomaria of the secondary formations of 

 Calvados, in which 53 species are mentioned as occurring in the Lias and the Lower and 

 Middle Oolitic systems. It is stated that they are exceedingly abundant ; but, on referring 

 to the Great Oolite species, 1 1 in number, we find, with one exception only, a repetition 

 of the following remarks appended to them : " One example ; two examples ; rare ; very 

 rare/' In fact, when describing the species which we have identified in that Memoir, we 

 seem, when stating their numbers, to be repeating the words of its author. 



1 Mem. Soc. Linn, de Normandie, vol. viii. 



