are so fine that they can scarcely be seen without the aid of a 
lens, and that they are finer and more numerous than those on 
the German specimens from the Weiss Jura gamma. 
Description. — The Yorkshire variety of Terebratulina 
substriata has the length, excluding the beak, about equal the 
width, in full grown specimens; but in some of the younger 
specimens the shell has its greatest width about the middle of 
the valve, and rapidly tapers towards the beak, giving the shell 
an elongated form, although the proportion of the greatest 
width to the length is the same as in the circular specimens. 
The shell is almost flat, with a slight convexity of both valves, 
the greatest thickness being about the centre of the shell, 
gradually becoming thinner towards the lateral margins. The 
larger valve has a slight wave-like depression commencing at 
the front margin, and gradually disappearing towards the beak, 
corresponding to a similar elevation on the smaller valve. 
The beak is almost straight; the foramen is large, and the 
deltidium is absent; there are no ear-like projections. Both 
valves are covered with very fine and numerous striae, which 
radiate from the beak towards the front margin of the valves. 
The sandy matrix of the deposit in which these shells are 
found is not favourable to the good preservation of the shell 
surface, so the striae often appear to have been worn. 
Dimensions. —The following is the greatest length, greatest 
width, and greatest thickness of the shells, given in mille- 
metres :— 
Large specimen . . IS m.m. 11 m.m. 5 mm. 
Medium specimen . . 10 m.m. 8 m.m. 4 m.m. 
Small specimen . . 7 m.m. 6 m.m. o m.m. 
An elongated specimen 10 m.m. 7*5 m.m. 4 5 m.m. 
Differences. — Terebratulina substriata var. Suffieldensis is 
easily separated from Zeilleria Hudlestoni, which occurs in 
the same bed, by being a much flatter shell, by the form of its 
beak, and foramen, also by its very fine striated surface. This 
variety differs from Terebratulina substriata var. alba by the 
extreme fineness of its striae; by its more rounded shape when 
full growm, and by the fold being only slightly developed, the 
shell being generally flatter. But the shell is more closely 
related to var. alba than to var. silicea. 
