Eidotheay a New Genus of Fossil Crustacea. 353 



dnncer Leachii, found in the London clay. Near the middle of 

 the shell, we observe three tubercles ; two internal, situated 

 within the concavities of two crescent-shaped eminences, and a 

 central one, placed in a depression between the convexities of 

 the same crescents. The central tubercle is the smallest, and 

 does not exhibit a reticulated, or granular surface ; and hence we 

 have not sufficient evidence for considering it an organ of 

 vision. The two lateral tubercles have their extremities broken 

 off, and have an annular appearance. We may consider these 

 bodies as eyes, especially as they are very nearly in the same 

 situation as the eyes of Limuli. Behind the eyes, the shell 

 becomes more convex than it is anteriorly, and has its surface 

 covered with numerous little spines. 



Tlie posterior part, or tail, is divided into segments ; but as 

 this part is imperfect, their number cannot be ascertained. Two 

 are represented in the figure ; and we have a detached fragment, 

 which indicates at least two more. The posterior margin of the 

 shell, as well as the anterior margins of the articulations of the 

 abdomen, are crenated. 



The inferior surface of this specimen is very indistinct, as it is 

 almost entirely imbedded in a mass of limestone. Indications of 

 feet are, however, visible. On the left side, near the posterior 

 part of the shell, there is a fracture of the limestone, in which a 

 part of a foot is observable. On the opposite side, the evidence 

 is much more satisfactory ; for we readily detect the three last 

 joints of a foot, and the terminal articulation forms a flat swim- 

 ming appendage. On one side we also observe, beyond the 

 margin of the shell, a number of granular bodies, which are of 

 the size and thickness of the finger, and appear to have been 

 either branchiae or parts of one. 



Length of the specimen, including a detached 



portion, which is not figured, . 9 inches. 



Breadth of the shell, . . .9 inches. 



Length of the shell, . . 6 inches. 



From the anterior margin of the shell to the 



eyes, . . . . 3| inches. 



Between the exterior margins of the eyes, 2 inches- 

 After a careful examination of this animal, I think we can 

 have little hesitation in considering it as belonging to the order 

 Entomostraca ; but it differs from any of the living genera of 

 that type, in several important characters. In the Limuli the 

 shell consists of two distinct pieces ; while, in our specimen, 

 it is entire, and exhibits no vestiges of any division. The body 

 of the Limuli terminates posteriorly in a long ensiform appen- 

 dage ; while, in the fossil animal, there is a tail consisting of 

 several articulations. It approaches nearer in structure to the 

 genus Apus ; but differs from it in the position of the eyes, 



VOL. III. 2 Y 



