354 



PALEONTOLOGY OF ILLINOIS. 



HETEROSCHISMA GRACILE, Wachsmuth, Nov. Sp. 

 / 9. 



Heteroschizma gracile, WACHSMUTH. 



Fig. 1, side view of a specimen. Fig. 2, horizontal section of the hydrospires. The 

 first figure enlarged three diameters, the other five. 



Body subclavate or irregularly pyriform, pointed at the base, 

 deeply excavated in the direction of the rays. From the foot of the 

 basals to the radial sinus the form is elongate-obconical, the sides 

 at the outer surface being straight or nearly so. The radials are 

 arched, with an angularity along the median line, which culminates 

 in the lips, but almost disappears on approaching the basi-radial 

 suture. Intermediate between the ambulacra there are five pyra- 

 mids, formed by the junction of the adjacent limbs of two contigu- 

 ous radials, without the assistance of oral plates. Four of them 

 are sharply pointed, projecting conspicuously above the level of the 

 summit; the fifth, the one containing the anal orifice, is a little 

 lower and truncate. Ambulacra, placed at the bottom of a deep 

 sinus; curving gently in an upward direction. The greatest width 



lacra, is readily septirated again into two sections, the one larger, more elongate, and 

 decidedly clavate, with nearly straight sides: the other smaller, with convex sides, and 

 somewhat larger and more prominent oral plates. Under these circumstances I would 

 be probably justified to drop Lyon's name entirely, but not wishing to overburden the 

 literature unnecessarily with names, I propose to apply his specific name to the smaller 

 form, as this is probably represented by Fig. 3 , and which will be known as Heteros- 

 chivma aUernatnrn. The larger type, which possibly may represent a more adult form of 

 the other, but more probably is a good variety or even an entirely distinct species, can be 

 distinguished as Heteroschisma altfrnatum var. elnngatum. which may be changed into 

 H. etongatum should the above characters be considered sufficient for specific distinction. 

 Bhumard's species is easily recognized by being much more flat-topped, that it has large 

 orals, and these abutting against the deflected upper ends of the limbs, and that the 

 radials generally possess along the lateral sutures, near the margin, one or more rather 

 deep, longitudinal grooves. I /yon. in asserting that the hydrospiral grooves were "prob- 

 ably capable of being compressed or impressed," made the mistake of taking specimens 

 of Cadaster pyramidat'is as the compressed, and those of Heteroschisma altematum as 

 the impressed condition of his species. It should be further remarked that none of the 

 Louisville specimens, upon which Lyon based his bicyclic base in Cadaster, show any 

 such structure. 



