78 The Ottawa Naturalist. [Oct. 



ginals seem to be of the same size, both rectangular, and the 

 plates of the supra-marginal row directly over those of the 

 marginal series. Further out on the arms, the plates are 

 pentagonal, those of the two rows alternating in position, and 

 dove-tailing, and the supra-marginals are smaller than the 

 marginals. One of the marginals, about half-way out on the 

 arm, is 1.25 mm. high and of about the same breadth. The 

 smaller triangular plates which cover the greater part of the arm 

 average about . 5 to .6 mm. in height. On one of the arms 

 (the one directed downward in the upper left-hand figure on 

 the plate), the small triangular plates seem to be arranged in 

 rows parallel to the axis of the arm, but the plates on the longer 

 arm seem to be more irregular, although a general arrangement 

 in rows can be seen. On this arm there are a number of very 

 small plates scattered about, especially on the top of the arm, 

 thus adding to the irregularity. The triangular shape of these 

 plates gives the arm a neat pattern, the plates making diagonal 

 rows backward and forward from the row of large plates along 

 the top of the arm. The madreporite, which is nearly circular 

 in outline, and 2 mm. in diameter, is in position, but slightly 

 tipped down at the inner side, in an interradius, and not far 

 from the centre of the abactinal side of the disk. The surface 

 is probably worn, for it appears perfectly smooth. 



In the fragment of the arm which is directed upward in 

 the upper left-hand figure and in the lowest figure on the plate, 

 the small plates are broken away, disclosing the ambulacral 

 plates. These plates, which are long and rather thick, seem to 

 be alternate in position. Two of the plates, well shown in the 

 lower figure, and indicated by an arrow, seem to be pierced by 

 pores near their proximate ends, two pores piercing each 

 plate vertically. Near the outer end of the more perfect arm 

 there is a space where a few of the small triangular plates are 

 missing, and here also the ambulacral plates can be seen from 

 the upper side. Each plate has a narrow keel on that side. 

 (See the middle figure on the plate, between the two brachiopods) . 

 Other details of the plates of the actinal side are unknown. 



This species seems tobe most nearly related to Palceaster 

 magnificus Miller.* to which species my attention has been 

 called by Professor Schuchert, who has most kindly loaned me 

 photographs of the type. Palaasterl magnificus is a large star- 

 fish (6 inches in diameter), found in the Waynesville division 

 of the Richmond formation in Ohio. Like Pal&aster? wilsoni, 

 it has two rows of large marginals and a row of large plates 



*Tcmr. Cincinnati Society Nat. Hist., vol. 7. ]>. 16, pi. 4, figs. 3, 3a, 

 1884. 



