54 platycrixid.e. — c ariocrin-us. 



Formation and Localities. 

 Niagara shale. — Luckport, N. America. 



Description of the Calcareous Skeleton. 



The Dorso-central Plate is quadripartite, the two smaller divisions being irregu- 

 larly quadrangular, and the two other, or larger ones, pentagonal. The inner angles, 

 where they meet in a central point, are minutely truncated to form the opening into the 

 columnar canal, and there is an elevated circular ridge, striated within, for the insertion 

 of the column ; elevated lines of granulje run from the centre to the terminal angles, and 

 other rows conform to the shape of the plate, while a few are sometimes met with, which 

 do not appear to follow any definite arrangement. 



The Perisomic Plates. — The first series, resting on the dorso-central base, consists 

 of six large plates somewhat conical in their centres, two of which are pentagonal, two 

 hexagonal, and the remaining two heptagonal. Eacli of these plates has several lines of 

 granulse, which in some specimens are confluent, and appear as ridges, radiating from a 

 convex point near their centres. These lines of granulee vary in number according to 

 the number of angles in each plate, the pentagonal plates having five, the hexagonal six, 

 and the heptagonal seven, so that a row of granulae runs from a central point, to each 

 terminal angle of the plate, and there meets with similar lines from the adjacent plates, 

 of either the first or second series. Other lines of smaller granulae conform to the shape 

 of the plate. Most of the larger granulae are perforated, and communicate obliquely 

 with the internal cavity which contained the digestive organs. The exact office in the 

 animal functions these pores were destined to perform, it is difficult to determine ; they 

 might have been for the extrusion of suckers, as in the recent echini, or for the purpose 

 of respiration. 



The arrangement of the granules is such as to add strength as avcII as to ornament the 

 plates, which are further strengthened by a number of ele\ ated radiating ridges on their 

 interior surfaces. 



Mr. Say describes the first series of plates (costals) as consisting of four pentagonal, 

 and two hexagonal ; they are however as represented in the dissected skeleton we have 

 given with the generic definition. The angles are sometimes not very decidedly defined, 

 but they are generally sufficiently acute to determine their number with certainty. 



