156 TRANS. ST. LOUIS ACAD. SCIENCE. 



Geological formation and locality — In the upper layer of the 

 Kaskaskia limestone: Evansville, Illinois, very rare. 



This species resembles very much a small Echinus in general 

 aspect, and is easily distinguished from others by its large trans- 

 verse diameter in comparison to the less vertical one. 



Pentremites Potteri, n. s. (Plate B, Fig. 4.) 



Body globose, nearly as w^ide as it is long. Pelvis very small, 

 not extending much beyond the articulating surface of the col- 

 umn. Fork pieces very short, only i of the entire length of the 

 body ; ornamented with stria running parallel to the suture, and 

 a little hook-like projection at the apex of the ambulacral field. 

 Deltoid pieces very large, covering | of the body, being laterally 

 framed by striated ridges which unite at the summit, leaving a 

 depressed triangular space between them ; this depressed surface 

 is divided by a central lineary crest into two equal parts and 

 ornamented with little granules. Ambulacral fields very small 

 and lineary, running nearly over the entire surface of the body. 

 Lancet pieces j as wide as the ambulacral field. Poral pieces 

 comparatively large for the size of the specimen, numbering 8 to 

 yV of an inch, and sunk below the level of the lancet piece. Mouth 

 central, round ; ovo spiracle apertures and anus very small, and 

 kept separate by the broad deltoid pieces ; they are surrounded 

 by the zigzag plicated integument in this as well as in other 

 species, but too diminutive and not near enough to the central 

 orifice for two of these openings to become united into one as 

 in other species. 



Dimensions — ^^ of an inch in width, and also the same in 

 length. 



Geological formation and locality — In the Burlington lime- 

 stone : at Burlington, Iowa. 



This fossil differs from Pentreiiiites Sayi, which it most resem- 

 bles, in that it is generally smaller, in that the pelvis is not con- 

 cave, the fork pieces not arched at the junction of the deltoid 

 piece, and this latter is not elevated and ornamented with trans- 

 verse striae, but depressed and granulated. 



I named this beautiful fossil in honor of Prof. W. B Potter of 

 Washington University. 



