4 Cincinnati Society of Natiiral History. 



a difference which makes any generic formula, containing descriptions 

 of these plates, subject to revision for each species to a certain extent, 

 as their sliape is by no means constant. The two basals are pentag- 

 onal, centrally excavated on their line of junction for the reception of 

 the column, this excavation in some cases being so widened outwardly 

 as to give rise to a deep cup in the center of the base, and to render 

 the basals carinate on their dorsal surfaces, while in other species it is 

 so small as to bareh^ admit the slender column. When united, the an- 

 terior side, at the end of the suture joining them, contains an angular 

 depression into which is fitted the corresponding face of the first radial 

 on that side. The faces of the basals at the opposite end of the suture 

 are similarly depressed for the reception of the azygos plate. The 

 first radial of the anterior side is heptagonal, much wider than high, 

 with a slight angle on the basal margin, the apex of which corresponds 

 with the line of junction of the basals. It is excavated on the opposite 

 face for the reception of the small second radials, the inner faces of 

 which meet on a line over its center, resting upon which, and their ex- 

 cavated inner faces, is frequently a small, sub-p^a-amidal, accessory 

 plate. The first radials joining the anterior one on either side are 

 hexagonal, twice as wide as high, and similar in form and size to the 

 anterior one, except that they join the basals by a single side instead 

 of two. They bear no accessory plates in an}^ of the specimens. 



The two posterior first radials are heptagonal, and have their ap- 

 plied inner margins excavated in such a manner as to receive the con- 

 vex sides of the azj'gos plate, above the outer extremit^^ of which they 

 unite, the suture being in an exact line with that of the basals. 



The second radials are ten in number, longer than high, irregularl}^ 

 quadrangular, so far as can be determined, Avith the longer side resting 

 upon the first radials, about one half of the central upper surface of 

 which they cover. They present slight differences in the ratio of the 

 height and breadth in the different species. Third radials twent}', sub- 

 quadrangular, about twice as wide as high. As there are four to each 

 of the first radials, two of them rest upon the second radials in the 

 center of their upper faces, while the other two have their inner 

 ends resting slightly upon the outer ends of the second radials, and 

 their outer ends joining the uncovered extremities of the first radials. 

 The form of these plates varies exceedingly in the different species, as 

 they are sometimes more than twice as wide as high, while in some of 

 the more conical species the two dimensions are nearly equal. 



The azygos piece is pyriform or sub-pyramidal, slightly angular at 

 its junction with the basals, and convexlv rounded to a point at the 

 opposite extremity, which is completely embraced b}' the first radials 



