106 BULLETIN 88, UNITED STATES NATIONAL MUSEUM. 



P. magnificus. It seems probable from the mature material studied 

 that the most differentiated species of PromopaJseaster, P. magniUcus, 

 passed through ontogenetic stages comparable to Hudsonaster, Meso- 

 palseaster granti, and Promopalxaster heUulus. 



P. magnificus at present stands alone in its beautiful diagonal 

 and longitudinal arrangement of abactinal plates. 



PROMOPALiEASTER WILSONI (Raymond). 



Plate 13, figs. 1, 2. 



Palaeaster? wilsoni Raymond, Ottawa Naturalist, vol. 26, 1912, p. 77, pi. 5, figs. 

 1^. 



Original description. — "The specimen is exposed from the abactinal 

 side, and preserves the greater part of one arm, the disk, and the 

 stumps of the other four arms. The diameter of the specimen, 

 when complete, must have been about 75 mm. (3 inches), and the 

 diameter of the disk is 20 mm. This is large for a starfish from the 

 lower Ordovician. The arms are quite convex, \vdth a gentle taper, 

 reminding one somewhat of the common recent starfish, Asterias 

 vulgaris, and as in that species, the arms were probably somewhat 

 flexible. The greater part of the abactinal side of the disk and arms 

 is covered with small convex, overlapping, V-shaped plates, which 

 are arranged with the point of the V directed toward the margins. 

 Along the crest of each arm there is a single row of larger plates 

 (radial column). These plates are quite large. and hexagonal in out- 

 Une near the disk, but become smaller, triangular, and alternate in 

 position farther out on the arm. There are tv/o rows of marginals 

 [supra and infra] these plates being larger and flatter than the other 

 plates, and covered with minute tubercles, wliich may be spine bases. 

 Close to the disk, the supramarginals and [infra] marginals seem to be 

 of the same size, both rectangular, and the plates of the supra- 

 marginal row directly over those of the marginal series. Farther 

 out on the arms, the plates are pentagonal, those of the two rows 

 alternating in position, and dove-tailmg, and the supramarginals 

 are smaller than the marginals. One of the margmals, 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 0.5 to 0.6 mm. in height. On one of the arms, 

 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 



