90 BULLETIN 88, UNITED STATES NATIONAL MUSEUM. 



rows being the most conspicuous) and separated from each other by 

 minute plates (becoming fewer on approaching the extremities of 

 the rays) . 



"The ellipsoid [supra] marginal tuberculiform plates number 

 about 12 [20 in mature examples] for each complete side of the rays, 

 and the central range [radial] is composed of a similar number, but 

 in form these plates are more circular. The disk between the ter- 

 minal central [radial] row of plates of the rays is crushed and struct- 

 ureless, except a slight elevation in the center. The terminal [proxi- 

 mal] plates of the marginal series are larger than the others of the 

 range. The madreporiform tubercle is of a spherical form and rela- 

 tively large, being nearly 2 millimeters in diameter, and is situated 

 at the axil of two rays. Both the tuberculiform plates and the 

 madreporiform tubercle have a granulated surface. 



"The ventral side is unknown. 



Emended description. A mature specimen measures: R=16 mm., 

 r=5 mm. ; R = 3.3r. 



Rays short, stout, tapering rapidly, and with very small inter- 

 brachial arcs. 



Abactinal area of disk unknown. Along the center of each ray 

 there is a radial column of small, highly convex plates bounded on 

 each side, at about the mid-length of the rays, by two columns of 

 smaller, strongly tumid, accessory plates. The radial column is, 

 however, not distinctly differentiated from those adjoining. Out- 

 side of these are the supramarginal columns, each with about 20 

 plates which are larger and more easily distinguished than the radial 

 plates. Dis tally the rays have only the ossicles of the infra- and 

 supramarginal and radial columns, with none of the accessory plates 

 as yet developed. 



Ambital area with one column of very small accessory plates 

 like the marginal plates on each side of them. These accessory pieces 

 disappear before attaining the apex of the rays. 



Madreporite spherical in form and relatively large, nearly 2 milli- 

 meters in diameter, and situated at the .axil of two rays between 

 two adjoining supramarginal columns. 



Formation and locality. From the Cataract formation, the basal 

 deposits of the Siluric system, near the reservoir in the city of 

 Hamilton, Ontario. Col. Charles C. Grant, after whom the spe- 

 cies is named, discovered most of the specimens, which are six in 

 number, two in the Spencer collection, now destroyed by fire, one 

 in the Redpath Museum of McGill University, Montreal, another 

 in the Geological Survey of Canada at Ottawa, and two in the 

 Grant collection, one of which has generously been donated to Yale 

 University. 



Remarks. This species is apparently closely related to M. bel- 

 lulus, which see for comparison. 



