>?6 On a fossil Crustaceans Animal. 



anterior part of some Trilobites. Eyes distinctly luiiated, 

 much depressed, and marked by co centric striae. Feet 

 four pair ; the two anterior composed of four or more near- 

 ly equal articulations, of which the terminal one is the small- 

 est and bluntly pointed, furnished with filaments, which from 

 their size and situation, are supposed to be branchiae. The 

 third pair is rather longer than the two proceeding, and 

 entirely destitute of filaments. The fourth or posterior pair 

 are placed near the junction of the head with the abdo- 

 men, and are larger in proportion to the body, than in any 

 living genus of Crustacea with which we are acquainted. 

 In the description above alluded to, these natatory feet 

 were mistaken for pectoral fins. As nearly as can be de- 

 termined from the faint and broken impressions of the up- 

 per part of these natatory feet, five articulations are 

 visible, of which the second is furnished on its anterior 

 edge, with two slight spines, and the last terminates in an 

 oval plate, as in the genus Portunus. The abdomen con- 

 sists of eleven distinct articulations tapering gradually 

 to the tail, a small part of which only remains. The abdo- 

 men presents no trace of a division into longitudinal lobes. 



DIMENSIONS. 



Total length 3-6 



Length of the head 1 



Breadth of do. - - - - 1-4 



Breadth of the body - 1-5 



Distance between the eyes - - - *G 



Cabinet of the Lyceum. 



Plate XXIX. 



The remarkable depression of the eyes, with other ap- 

 pearances, lead to the belief that this fossil is merely an im- 

 pression, and that this impression has been moulded from 

 the upper or dorsal surface of the animal. 



