526 GEOLOGY OF OHIO. 



the pygidium. The diagonal anterolateral furrows soon disappear pos- 

 teriorly, and alone; the posterior margin the pygidium retains its convexity, 

 no trace of a concave border being present. 



Illcenus Madisonianus, Whitfield. 



(Pla'.e 26, Figs. 1, 2, varieties; Plate 27, Figs. 7, 8, 9, 10.) 



In the limestone at Brown's quarry occur small heads and pygidia 

 showing all the characteristics of this form except as regards size. In 

 the " Orthoceras block" from the base of the Clinton at Huffman's quarry 

 the same form, but of typical size, was common; the movable cheeks being 

 at times still in position, a rare occurrence elsewhere in the Clinton. Inform 

 the pygidia resemble those of I. insignis. but are less elongated, taking in 

 this respect a position intermediate between I. ambiguus and I. insignis; 

 this is especially true in the Orthoceras block specimens. The pygidium 

 differs from both species, however, in possessing a distinctly concave bor- 

 der. The hypostoma and rostrum of this species are known from the 

 same " Orthoceras block." 



At Huffman's quarry, in the middle limestones, was found a specimen 

 also with a concave border but decidedly elongated, more equally and 

 strongly convex, with the greatest elevation at the anterior third of the 

 pygidium. This form maybe designated as var. elongatus. ( Plate 26, figs. 

 1, a,b.) 



Another form, found at Stoltz's quarry in the limestone, presents also 

 a concave border, but the general form of the pygidium is broad and 

 much lessconvex; as in the last form, the greatest height is well towards the 

 anterior : in this case near the anterior margin. It may be called 

 var. depressus (Plate 26, figs. 2, a, b.) 



These forms can scarcely be said to have the value of distinct va- 

 rieties, yet it may be useiul in tracing out the phylogenetic variations of 

 Illaeni to have a name for some of the intermediate forms. Illcenus 

 ambiguus, III. insignis and Illcenus Madisonianus form a group of very 

 closely allied species. Ill cuniculus seems to belong to the same group. In 7 

 the cabinet of Dr. E. N. S. Ringueberg I noticed various species or forms 

 of Illcsnus. In the Niagara shales from L,ockport, New York, forms like 

 ///. Madisonianus. III. cuniculus, a form near ///. ambiguus, and ///. Dayton- 

 ensis were seen. It has already been noted on a former occasion that 

 various species of Illcenus exist in New York Upper Silurian rocks. All 

 these still await a careful revision. 



Calymene Vogdcsi, Foerste. 



I Plate 25, Fig. 25 ; Plate 27, Figs. 12, 13, 16 ; var. Plate 25, Fig. '.'4 ; Plate 27, Figs. 14, 15.) 



This form, first described in the second volume of the Bulletin of 

 Denison University, is scarcely more than a varietal phase of a poly- 

 morphus species including a large percentage of the forms found in mid- 



