THE LOWER LIAS OF KEYNSHAM. 
53 
A distinct mutation is produced when the ribs become 
coarser and more rounded and almost die out under the beak. 
This is the case in the shell which occurs so plentifully in 
the upper Semicostatus beds. In this Rhynchonella^ there 
are four broad ribs on the mesial fold and two or three only 
on each flank. (In most specimens, which show the junction, 
this appears to be flat but, owing to the crushed state in 
which the shells occur, it is impossible to definitely assert 
that this is an original character.) This variety we have 
named Eh. calcicosta^ var. semicostati on account of the 
stratigraphical position which it occupies. This form may 
be identical with Quenstedt’s T. belemnitica {Dcr Jura t. 8, 
f. 5) which agrees in the general character of the ribbing, 
but differs in size and (probably) in its flatness. 
It is interesting to notice that Eli. belemnitica^ Quenst. 
occupies, almost exactly, the same horizon in Germany that 
our shell does at Keynsham, being associated with the first 
entrance of Belemnites in any abundance. In the German 
Lias, Eh. plicatissima first appears in the Arietes beds, whilst 
Eh. calcicosta does not occur until the Oxynotus zone and 
is not common until the Spirifer bank (lower Jamesoni beds). 
Eh. triplicata^ Quenst. This is a member of the Eh. 
variabilis group with deep sinus, bounded by two strong 
folds and an almost smooth test under the beak. 
Echinids. 
Pseudodiadema^ n. sp. This occurs abundantly in the 
Echinid clay, at the top of the Psilonotus zone. From the 
very numerous fragments which we have obtained we are 
able to give the following detailed description of the species. 
Diameter *6 inches (to greater). 
Diameter of apical disc T5 inches. 
Apical angle of interambulacral zone 55°. ’ 
Apical angle of ambulacral zone 18°. 
Pores uniserial. 
