328 GASTEROPODA OF THE INFERIOR OOLITE. 



and in the lower zones of the Inferior Oolite in Dorset, especially at Bradford 

 Abbas. Rare at Duston. 



The two species next described are closely related, and indeed Onustus 

 acuminatus is little more than a variety. 



260. Onustus acuminatus, sp. nov. Plate XXVII, fig. 3. 



Description .• 



Height restored . . . .22 mm. 



Width . . . . .26 mm. 



Opening of spiral angle . . . 40°. 



Mean spiral angle . . . .75°. 



Shell pyramidal, imperforate ; spiral angle extremely concave. Number of 

 whorls about nine ; the apical whorls are flat, but with the increase of the spiral 

 angle become much more excavated, so much so that the spire seems as if it 

 scarcely belonged to the three anterior whorls. The ornaments are similar to 

 those in some of the varieties of 0. pyramidatus, the costae being fully developed 

 to the very edge of each whorl. The base is flat, smooth, or with slight radial 

 lines of growth, and there is scarcely any trace of an umbilical excavation. 



Relations and Distribution. — This form exhibits the concave spiral angle to a 

 far greater extent than any species of Onustus in the Inferior Oolite of this 

 country, in this respect somewhat resembling Onustus papyraceus,JI6b. and Desl., 

 from the Callovian of Montreuil-Bellay. It is quite possible that where the spire 

 has been lost there are no means of distinguishing 0. acuminatus from the more 

 common 0. pyramidatus. 



Rare in the Murchisonai-zorie at Bradford Abbas, and probably occurring on 

 the same horizon at one or two other places in North Dorset. There are two 

 specimens from the Inferior Oolite of Nailsworth, now in the Jermyn Street 

 Museum, which may be referred to 0. acuminatus. 



261. Onustus, species or variety. Plate XXVII, figs, ha, bb. 



Description : 



Height . . . . .16 mm. 



Width . . . . .23 mm. 



Spiral angle ..... 82°. 





