FOSSILS OF THE NIAGARA GROUP. 357 



The two following species, both on account of their form and the great 

 number of small polygonal plates, may prove distinct from the typical 

 forms of this genus : 



HOLOCYSTITES OVATUS, N. S. 

 PLATE XII, FIG. 2. 



Body from the summit to near the base symmetrically ovoid ; a little 

 depressed at the central aperture, and elevated at the eccentric one ; 

 sides regularly curving ; composed of more than ten ranges of alter- 

 nating polygonal plates, which are as wide as long, or wider, elevated 

 in the centre, and coarsely granulose. 



A specimen of one inch and three-fourths in length, and perfect at the 

 base, shows ten ranges of plates between the broken lower margin and 

 the summit aperture. The greatest diameter of the same individual is 

 one inch and a half, while it is about half an inch in diameter where 

 broken off, and there have probably been two or three ranges of plates 

 below that point. A section below the summit is very broadly elliptical, 

 and this has probably been the original form. 



This species differs from H. scutellatus in being a more robust form, 

 with less elongate base, and with plates larger, more equal in size, and 

 prominent in the middle ; while the larger plates in that species have 

 central nodes from a nearly flat surface. 



Formation and Localiti/. — In the limestone of the Niagara group at 

 Waukesha, Wisconsin. 



HOLOCTSTITES SCUTELLATUS, N. S. 

 PLATE XII. FIG. 1. 



General form of body ovoid ventricose, abruptly attenuate below, and 

 swelling above into an ovate outline ; composed of twelve or more 

 ranges of plates, which are somewhat irregularly disposed, there 

 being in the upper part a distinct arrangement of one large polygonal 

 plate surrounded by smaller ones, this large plate having a node in 

 the centre. The summit is broadly rounded, with evidence of one 

 large subcentral aperture, and a depression indicating a second 

 aperture. Surface of plates strongly granulose. 



The ventricose ovate form and abruptly attenuate base are distin- 

 guishing features of this species. Though the H. cylindricus sometimes 



