l6o TRANS. ST. LOUIS ACAD, SCIENCE. 



zontal in adult specimens. Fork pieces a little longer than 2 the 

 entire length of the body ; the base portion is horizontal ; the 

 prongs sloping from the projecting margin of the ambulacral field 

 very abruptly in their lower portion, but more gently in their up- 

 per part towards the sutures of the adjoining pieces, giving thus 

 to the interambulacral space a very depressed triangular surface 

 up to the lower obtuse angle of the deltoid piece, from whence 

 the interambulacral surface becomes level upwards, with two 

 acute points downwards running lateral to the ambulacral fields- 

 Deltoid pieces large, lanceolate, not depressed, and project with 

 their acute rostrum to the summit above the body. Ambulacral 

 fields nearly as long as the body itself, broad and depressed 

 in the centre. Lancet pieces a little more than half the width of 

 the ambulacral field, and concave in transverse section. Poral 

 pieces numbering 8 to -j-^ of an inch. All apertures on the sum. 

 mit large, but closely arranged. Surface smooth and not orna- 

 mented. 



Dimensions — Vertical height ih inches; greatest width a little 

 above the apex of the ambulacral field, nearly the same. 



Geological formation and locality — Kaskaskia limestone : 

 Evansville, Chester, and other places in Illinois, not very abun- 

 dant. 



This species is one of the largest of its kind, and often reaches 

 the size of nearly 2 inches. It much resembles Pentretnites sul- 

 caius, but has a more horizontal base, is longer, and also more 

 conoidal. It differs from P. Missouriensis (Swallow) in having 

 a smooth surface as well as in the acute projecting rostrum of the 

 deltoid piece. 



