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former species. It is nearly semiorbicular, lunated posteriorly, and 
terminating at the sides in an acute angle. The body, which has only 
five transverse plates, is remarkably short ; its sides going off directly 
from the head, and meeting speedily at an obtuse angle. From this 
point proceeds the tail of the animal, which is of a greater length than 
both the head and body. The structure of the tail may be here so far 
made out, as to enable us to ascertain that it is formed by a long 
central spine-like process, on each side of which a membrane has 
been evidently extended, wider than the process itself. 
This fossil appears to be the same with Monoculites lunatus of Mr. 
Martin, Plate 45, Fig. 4, who supposed it to approach nearer, in size 
and figure, to the Monoculus apus, than to any other known recent 
species of that genus. 
The opinions respecting the analogue of the trilobites have been 
very different. Some have supposed it a testaceous animal, and some 
have imagined it to be a coleopterous, whilst others have conjectured 
it to be an apterous insect. Guettard and Davila have placed it among 
the crustaceous animals. Linn ecus, Mortimer, and Wilke, think it 
should be placed among the monoculi. Several writers have con- 
sidered it as proper to place it among the bivalve shells ; and Leigh, 
Hist, of Lancashire, Tab. vii. f. even regards it as a portion of a 
nautilus ; Scheuchzer supposed that it might have been a patella ; 
and Bruckman speaks of it as a polype. We must content ourselves, 
I believe, with allowing that no animal resembling it is known. Its 
surface, however, viewed with a lens, confirms the opinion of M. 
Walch and others, who have supposed it to be an animal of the crus- 
taceous kind ; the roughness resulting from the numerous little pits 
and risings appearing to be very similar to that of the crust of the 
crab, lobster, &c. 
Plate XVII. Fig. 19, is the fossil remains of some crustaceous 
animal, which are frequently found with the trilobite in the Dud- 
ley lime-stone. The head part of the animal appears to have 
