Description of a new Trilohite. 169 



AsAPHUs Platypleurus. — Green. 



Clypeo? corpore convexo; costislaiis, pJanissimis ; parte marginali 

 vix membranacea ; cauda rotundata. 



I have seen the lower portions only of this trilohite, hut the char- 

 acters which they present are sufficiently striking and peculiar to dis- 

 tinguish it from the other species. Ten articulations of the caudal 

 extremity of the animal still remain in a good state of preservation. 

 The middle lobe of the back is scarcely elevated above the plane of 

 the lateral lobes, and as often occurs in the Asaphs ; it has a regular 

 conical appearance, the apex of the cone forming the terminal joint 

 of the tail. Two or three of the last articulations in our specimen 

 are somewhat obliterated. The ribs both of the sides and of the 

 middle lobe are broad and very much flattened ; their upper surface 

 is entirely smooth, except a slight sulcus near the lower edge of the 

 costal arches. The grooves or furrows formed by the joints are nar- 

 row and not very deep. The tail is rounded, and the membranace- 

 ous expansion or border which is found in most of the Asaphs, is in 

 this species but very little developed. In one side it is mutilated in 

 our fragment. The body of the animal projects considerably above 

 the rock in which it is imbedded, and is therefore very unlike the 

 depressed forms of most of the Asaphs; indeed, at first sight I took 

 it for a Calymene. 



The rock in which the A. Platypleurus is mineralized, is composed 

 of hard, compact, black limestone. A small fragment of another 

 Asaph reposes near the one just described. It may perhaps be the 

 remains of the young of that species, though the ribs are much more 

 rounded. There is also the joint of a small encrinite adhering to 

 one of the costal arches. 



I am indebted to my young friend, Dr. R. M. Jackson, for this 

 and some other trilobites found by him in Huntington County, Penn. 



Vol. XXXII— No. 1. 22 



