224 



TRANSACTION!; OF THE 



ON THE PENTREMITES REINWARDTll, A NEW FOSSIL; WITH 

 REMARKS ON THE GENUS PENTREMITES (SAY), AND ITS 

 GEOGNOSTIC POSITION IN THE STATES OF TENNESSEE, 

 ALABAMA AND KENTUCKY. By Gerard Troost,M.D., Professor 

 of Chemistry, Mineralogy and Geology in the University of Nashville, 

 Tennessee ; Member of the Geological Societies of France and Pennsylva- 

 nia, &c. &c. 



The organic remains which form the subject of this 

 memoir, we place for the present in the genus Pentre- 

 mites, established by our worthy countryman Thomas 

 Say, although it differs perhaps in some essential points 

 from that genus, but its fossil state (it being entirely 

 changed into calcareous spar) makes it difficult to ascer- 

 tain its minute organs. 



It is composed, like that genus^ of five ambulacra^ ra- 

 diating from the summit and terminating about the mid- 

 dle of the intermediate fields or scapulae. No visible 

 sutures separate the pelvis from the scapulae, nor is the 

 pelvis or base divided into different parts, but a suture 

 runs from the tip of the scapula to the lower extremity 

 of the pelvis. 



The ambulacra are not lanceolate as in the other already 

 described species, but they are rounded at the base ; nor 

 are the pores placed in the same manner on the tenta- 

 GulsB. We annex a magnified representation of an am- 

 bulacrum (see pi. 10, fig. 11), from which it appears 

 that a zigzag line divides it longitudinally in two parts. 

 Strise in a more or less inclined direction run in an alter- 

 nating order from this line to the margin, and a small 

 pore is visible half way between the line and the margin. 

 This representation gives at the same time the form of 

 the ambulacra. No aperture is visible where these am- 

 bulacra join together at the summit. Nor are the five 



