*267 



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 ascer- 

 tain 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. Mar- 

 tin, 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. Linnaeus, Mortimer* and Wilke, think it 

 should be placed among the monoculi. Several writers have considered 

 it as proper to place it among the bivalve shells; and Leigh, Hist, 

 of Lancashire, Tab. vn. 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 crustaceous kind ; the rough- 

 ness 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 Dudley lime-stonCo 

 The head part of the animal appears to have been separated and r&~- 



