CRUSTACEA. 123 



"C. marginalis, Buckler, with a broad margin : eyes large, semi-oval; middle 



lobe entire, convex, smooth abdomen ' 



'■'■Locality. Near Ithaca, in a boulder. This has a much less prominent 

 front than the Rowi, a deeper groove between the eye and middle lobe, and 

 the tubercle which nearly joins the lower angle of tin- eye is much smaller." 



It was subsequently ascertained that the boulder referred to by -Mr. Conrad, 

 was a mass of the Tully limestone, which had been transported many miles south 

 of the outcrop of that formation. For this reason the species of Proetus which 

 has been frequently found in the outcrops of this rock upon Seneca and Cayuga 

 Lakes, in Onondaga county, and elsewhere, was regarded in the published de- 

 scriptions of 1861 and 18G2 (rid. loc), as probably specifically identical with Mr. 

 Conrad's original. This reference was without doubt correct, but the more com- 

 plete material illustrating this fossil and its allies in adjoining formations, which 

 has accumulated since the publication of these descriptions, has afforded eon- 

 elusive evidence that P. marginalis, Conrad, is identical in every specific detail 

 with P. Rowi, Green. 



The points of difference in these forms, as stated in the description quoted 

 above, depend solely upon the condition of preservation of the specimens. As 

 the individuals of P. marginalis occurring in the State of New York are pre- 

 served in limestone, the surface has more nearly retained its normal rotundity, 

 and the glabella is less extended anteriorly than in the usually somewhat flat- 

 tened examples of P. Rowi from the Hamilton shales. The specimens from 

 the Tully limestone show that the glabella encroaehes slightly upon the frontal 

 margin, as in normal examples of P. Rowi. 



The depth of the groove between the eye and the glabella depends entirely 

 upon the degree of flattening of the specimen ; and the size of the occipital 

 lobe varies with the absence or retention of the crust. 



After a careful examination of all the representatives of P. marginalis at 

 hand, it seems impossible to associate any characteristics by which it can be 

 distinguished as even a variety of P. Rowi, and the foregoing description of the 

 latter species will apply in every respect to Mr. Conrad's P. marginalis. 



This fossil occurs in the Tully limestone, in intimate association with Pha- 



