Ii;8 TRANS. ST. LOUIS ACAD. SCIENCE. 



in the centre, but for each half a rather convex surface. Lancet 

 pieces a little more than ^ of the width of the ambulacral field. 

 Poral pieces rather thick, numbering 8 to y\y of an inch. All 

 apertures on the summit large and closely arranged. The whole 

 surface of the body is very distinctly ornamented with little Yur- 

 rows running parallel to the sutures. 



Dimensions — Vertical height as well as transverse diameter, 

 i.e. a little above the apex of the ambulacral field, ly'^ inches; 

 vertical distance from centre of pelvis to apex of ambulacral areas 

 j\ of an inch, length of ambulacral field i^\ of an inch, width of 

 same y\ of an inch. 



Geological formation and locality — In the Kaskaskia limestone : 

 common at Chester, but rare at Evansville, Illinois. 



This species resembles veiy much, at first glance, Pentremites 

 robustus of Lyon, but differs in the smaller number of poral 

 pieces, which are lor in lyV of an inch of "Lyon's" species, 

 whereas in the one under consideration there are only 88 in the 

 same space. 



Pentremites Chesteriensis, n. s. (Plate B, Fig. 8.) 



This species seems to be a variety of either robustus ox hemis- 

 phericus^ but it appears so frequently under the same dimensions 

 that I do not hesitate to regard it as a separate species from any 

 of the foregoing. Its body is oval. Pelvis as large as \ the trans- 

 verse diameter, forming a little cup. Fork pieces long ; the base 

 portion slopes abruptly, whereas the upper parts slope gently to 

 the sutures of the adjoining pieces, giving thus to the interambula- 

 cral fields an almost even surface, or one very slightly depressed. 

 Deltoid pieces lanceolate and as long as \ of the entire length of 

 the body, projjecting with their acute angles slightly above the 

 apertures on the summit. Ambulacral areas as long as | of the 

 vertical diameter, narrow and depressed in the centre. Lancet 

 pieces \ as wide as the ambulacral field. Poral pieces large, 

 numbering 6 to ^^ of an inch. Apertures on the summit small 

 and closely arranged. Surface ornamented with very fine striae. 



Dimensions — Vertical height \^^ inches, transverse diameter 

 y8^ of an inch, length of ambulacral field ^^ of an inch, width ^^ 

 of an inch. 



