20 
THETIS major. 
TAB. DXIII.—figs. 1. to 4. 
Spec. Cuar. Convex, orbicular; posterior edge 
rather angular ; beaks small. 
Orren two inches wide, and nearly as much long; ex- 
tremely thin: the surface is plain and smooth, except 
numerous longitudinal rows of very minute rising punc- 
tums, probably epidermis: the beaks are small, sharp, 
and curved close together. 
The size and a less degree of convexity are almost 
the only points in which this differs from the following. 
The most perfect specimens we have met with, were col- 
lected at Blackdown, in 1812, by Miss E. Hill; they are 
siliceous casts imbedded in Sandstone, and do not appear 
to have undergone any violence, as they are regular and 
precisely similar in shape. (See fig. 1.) The same species 
is very abundant in the soft micaceous Sandstone near 
Devizes, where there are only casts, which, in common 
with the other fossils of that place, are much and various- 
ly distorted (figs. 2.5. &4.). For these we are under 
obligations to Miss Gent. It occurs also at Earlstoke. 
