No. 186.] 



19 



The preceding figures (with one exception) illustrate some of the numerous 

 varieties of form which occur in the rocks of the Lower Helderberg group, in 

 which, and the Oriskany sandstone together, we know nearly fifty species. 



The following species from the Upper Helderberg limestone ofiers an illus- 

 tration of the spiniferous forms : the specimen is one of medium size of the 

 P. dumosum (Conrad). The larger individuals of this species are sometimes 

 more thickly set with spines ; and in a single individual before me, some of 

 these spines are two inches in length. 



Plattcekas dumosum. 



There are about fifteen species of this genus already known in the 

 limestones of the Upper Helderberg group, some of them approach- 

 ing in character to those of the Lower Helderberg limestones ; but they 

 never acquire that degree of development, nor the prolific variety of 

 form and number of individuals, which meet us in the latter group of 

 strata. 



We already know several species in the Hamilton group, and amorg 

 them one spine-bearing form ; but the individuals are comparatively 

 rare, and they form but an inconspicuous feature in the fauna of the 

 period. 



In the Chemung group, where the calcareous element is greatly subor- 

 dinate, and the mass consists principally of sands and clays, these forma 

 are exceedingly rare or almost entirely wanting. 



In the Carboniferous limestones of the Mississippi valley, we already 

 know some seven or eight species, and a larger number will doubtless be 

 found during the progress of investigations. 



