ONUSTUS. 327 



Description. — The following are the proportions of the type specimen : 

 Height . . . . .17 mm. 



Width . . . . .23 mm. 



Spiral angle ..... 80°. 



The spiral angle may be said to range from about 80° to 86°, and is slightly 

 concave to nearly regular. Whorls six to seven in number ; they are irregularly 

 concave and more or less overlap, the line of junction being usually slightly 

 crenulated. The costae are numerous, prominent, straight or slightly sinuous, the 

 intercostal spaces being roughened by closely-set lamellae, which give a scaly 

 appearance. This is much more conspicuous in some specimens than in 

 others — a circumstance which may be attributed to differences in mineralisa- 

 tion. 



The margin of the base is but slightly crenulated, the peripheral edge being 

 almost plain, and so thin that the outline is seldom perfectly preserved in any of 

 the specimens. The base has very fine radial lines, with occasional traces of spiral 

 ones, but is otherwise smooth ; it is only slightly concave, rising again towards 

 the columella. There is a semicircular umbilical excavation of moderate 

 dimensions, but no umbilicus. Aperture very oblique. 



Varieties. — Of those to which it may be necessary to draw particular attention 

 the most frequent is one which occurs in the Dogger and elsewhere, and which 

 was mistaken by me for 0. ornatissimus, d'Orb. {vide ' Geol. Mag.' ut supra). 

 In this form the costae are very short, and do not meet, so that the imbrication or 

 overlap of the whorls is very conspicuous ; spire conical. This variety might be 

 known as semicostata. 



Another variety from the Dogger is a very conical form, presenting the usual 

 characteristics of 0. pyramidatus, especially the flat and scarcely excavated base, 

 but is absolutely devoid of costae. Whether or no this is a trick of mineralisation 

 seems uncertain. This may be known as var. incostata. 



There are also in the Murchisonse-zone of Bradford Abbas and Halfway House 

 some megalomorphs, which it would seem proper to regard as referable to this 

 species, the more so as they exhibit the characteristic base ; one of these must 

 have measured 40 mm. in width. These seem to answer to Eugene Deslongchamps' 

 description of Onustus lamellosus, d'Orb. (vide * Bull. Soc. Linn. Norm.,' vol. v, 

 p. 133). 



Relations and Distribution. — Onustus pyramidatus may be regarded as the most 

 abundant representative of the genus in the Jurassic rocks ; distinguished from 

 0. heliacus, d'Orb., by its larger habit, more closely-set costae, and more scaly 

 surface, and also by the greater smoothness of the basal border. The same 

 features will also serve to distinguish it from 0. ornatissimus, d'Orb. 



It is not common anywhere, but occurs mostly in the Yorkshire Dogger 



