120 PAPERS, ETC. 
striate. Interior of the ventral valve shews two produced 
teeth, between which, under the deltideum, is a small 
central ridge, on either side of which are muscular depres- 
sions; beyond which are the impressions of the larger 
muscles. About the middle of the cavity of the valve, 
commence striated ribs, which become more produced as 
they approach the inner front of the shell, terminating at 
the margin of the valve in small bosses or knobs. The in- 
terior ofthe dorsal valve has a deep frontal margin, com- 
prising nearly half the area of the valve, chiefly occupied by 
a series of deep grooves, which received, when closed, the 
bosses of the ventral valve; where the grooves cease, a 
flattened striated band oceurs. Within the margin is an 
elevated ridge, with granulations, united at the top by a 
straight ridge, forming a bridge over an elongated visceral 
cavity, and at the bottom by a broad septum. The inner 
portion of the valve is occupied by a calcified supra-mem- 
braneal disk, divided into two lobes by the central septum. 
Obs. This beautiful species of Thecideum is the only 
one which presents so peculiar a frontal margin. It is very 
rarely found perfect; only two specimens shewing the 
supra-membraneal disk having been obtained. The usual 
forms of the less perfeet dorsal valves may be seen in 
varieties fig. 4—5. On the interior of the dorsal valves 
are large punctuations. The outer front of the ventral 
valve, when perfect, is also punetuated ; but when worn, 
the internal striated ribs appear. 
From the inferior oolite of Dundry. 
THECIDEUM FoRBESEL.—Moore—Plate 3, fig. 8—10. 
Shell transversely oval, depressed, smooth; deltideum 
short, raised, triangular; area flat; hinge line straight; 
