NIAGARA LIMESTONE OF CHICAGO, ILLINOIS. Ill 



This species has the calycinal form of M. Christyi Hall sp., but in that species the bifurca- 

 tion of the rays takes place at more than two thirds the distance from the stem to the arm 

 bases, while in ours the bifurcation is very little above the mean height of the cup. The 

 marked sulcations between the pairs of arms, especially on the azygos side, existing both 

 on the exterior and in the cast, constitute the most marked peculiarities of our species so 

 far as at present known. 



Holocystites sphsericus W. and M. 



Body spheroidal, slightly produced on the basal side, composed of about twelve ranges of 

 polygonal plates rather irregularly disposed, and varying both in form and size, each plate 

 apparently being indented in the middle. 



The only specimen of this cystidean in our possession, is a mould of a portion of the 

 exterior, which, when first discovered, was nearly a hollow hemisphere. Its generic rela- 

 tions are not satisfactorily shown, but the great number of ranges of plates seems to ally 

 it to Holocystites, a genus so abundantly represented in the neighboring State of Wis- 

 consin, while the globoid form distinguishes it from any of the species recently described 

 by Prof. Hall. 



Conocardium ornatum W. and M. 

 Plate II. figure 15. 



Shell small, very ventricose ; beak a little nearest the anterior end, projecting beyond the 

 hinge line and greatly incurved ; surface arching from the beak to the ventral side in nearly 

 the form of a semicircle. The truncation, or separation of the rostrate slope from the 

 anterior aspect of the shell forms a posterior angle of about 75 with the cardinal side, and 

 an anterior angle of about 55 with the plane separating the two valves. This truncated 

 plane rises into a crest or elevated rib on the surface of the valve, a little anterior to which 

 and forming an angle of about 20 with it, is another, double-crested, but feebler rib. The 

 remainder of the surface is marked by regular radiating raised striae. Decussating these 

 and the ribs is a set of numerous sharp raised concentric striae stronger than the radial 

 ones, the two sets producing a beautifully cancellated surface. The concentric striae extend 

 over the rostrate slope of the valve, but become there feebler, more crowded, and less 

 rigid. 



Length of the shell, .41 ; dorso-ventral diameter, .30 ; thickness through both valves, 

 .32 ; distance from beak to anterior end, .16 ; to rostrate extremity, .25 inch. 



Porcellia senex W. and M. 

 Plate III. figure 6. 



Shell small, consisting of one and a half or two very rapidly enlarging, detached whorls, 

 which are somewhat oblique in the young, but afterwards continue very nearly in one 

 plane. Toward the aperture the shell is flattened and sub-nodulous on the dorsum, the 

 nodes consisting of a larger and a smaller, alternating with each other in a line along 

 the middle of the flattened surface. The angle by which the dorsal surface unites with the 

 upper side of the whorl is rounded; the other angle" is considerably sharper. The aperture 

 is not expanded, but presents a deep sinus extending across the dorsal side. Apex of spire 

 depressed much below the level of the upper side of the outer whorl. 



