1 4G PROCEEDINGS OF THE ACADEMY OF 



Second anal oblique, wider (obliquely) than its diameter in the direction of 

 its vertical axis, irregularly pentagonal, resting with its base upon the upper 

 truncated side of one of the subradials, and its left side against one of the 

 first radials ; while its upper right margin connects with another, and its sinu- 

 ous oblique superior side forms the under margin of the anal opening. Pec- 

 tinated areas situated in deep excavations, those at the angles of the basal 

 and subradial pieces largest, and obscurely trilobate ; the smaller ones at the 

 angles above oval or subcircular. Surface ornamented with strong radiating 

 costse extending from the centre to each of the sides of the plates, and all 

 widening from the centre outwards. Sutures distinctly furrowed, even on 

 the truncated under side of the base. 



Length - 72 inch ; breadth about O66 inch. 



This species will be at once distinguished from P. conicus, of Billings, the 

 typical species of the genus, by its oval instead of obconic form, (being widest 

 a little below the arms, and rounded in above), and the strong radiating cos- 

 tse of its plates. We know of no other form with which it need be compared. 



Like the typical species, its free arms commenced with the second radial, 

 and were evidently slender, and nearly cylindrical, or a little compressed late- 

 rally, and provided with a very small furrow above. We have not seen the 

 column, but it appears to have been large at its connection with the base. 



Nor have we been able to see the structure of the small crown occupying 

 the narrow space within the area surrounded by the arms, but it seems to 

 consist of about three or four comparatively large plates. 



Locality and position. Oswego, Kendall Co., 111. Cincinnati Group,* Lower 

 Silurian System. 



POROCRINUS PENTAG0NIUS, M. & W. 



Body pentagonal-obovoid, being more or less rounded above, and tapering 

 at an angle of about sixty degrees from the middle of the prominent sub- 

 radials to the summit of the column ; base forming about one-fifth of the 

 entire height, and having the form of an expanding pentagonal basin, with 

 flattened sides ; basal pieces pentagonal, and nearly twice as wide as high. 

 Subradial pieces as long as wide, and equalling nearly half the length of the 

 body. the only one visible on all sides in our specimens, hexagonal in form ; 

 each prominent in the middle, from which point a well-defined ridge radiates 

 so as to connect with similar ridges on each of the surrounding plates ; the 

 ridges passing laterally and upwards intersect the sides of the plates, but the 

 one passing downwards from the middle of each subradial coincides with its 

 central inferior angle, where it connects with a corresponding ridge extending 

 up the sutures between the basal pieces ; the arrangement of the ridges being 

 such us to divide the surface into a series of large triangular, slightly concave 

 areas, in which are placed the pectinated openings. These openings at the 

 corners of the basal and subradial pieces consist of about twelve of the linear 

 fissures to ea< h plate ; those at the junction of the plates above smaller, with 

 a proportionally smaller number of fissures. Form and arrangement of the 

 anal and radial pieces, as well as of the arms, unknown. 



Surface finely granulo-striate, the granules being ranged in lines parallel to 

 the ridges, particularly on the ridges below the middle of the subradials, so as 

 to present, as seen under a good magnifier, a finely substriated appearance. 



Column rounded, and expanding rapidly upwards near the base, where it is 

 composed of very thin segments with minutely crenated edges ; farther down 

 the segments are proportionally thicker and more coarsely crenate. 



Length of body, 0*43 inch ; breadth at the middle of the subradials, 0-40 

 inch. Breadth of column at its connection with the base, 0-15 inch ; do. - 72 

 inch below, 0-05 inch. 



* See Note at the end of this paper. 



C A g. 



