NATURAL SCIENCES OF PHILADELPHIA. 169 



This species is related to Platycrinus Aviericamis, of Owen and Slmraard, 

 with which it agrees in the size and form of its body. Its surface sculpturing, 

 iiowever, is somewhat different, that species having merely a nodular ridge 

 running along the lower and lateral margins of the first radial plates, and two 

 others starting from the lower lateral angles and converging to the sinuses in 

 the middle of the upper edge, with little isolated nodes on the intermediate 

 spaces ; while in the species under consideration there are merely three some- 

 what nodular ridges, parallel to the basal and lateral margins of these plates, 

 with more or less granules in the central region As such markings, however, 

 are subject to some variation in individuals of the same species of this group, 

 we should not have regarded the differences mentioned of sufficient importance 

 to warrant the establishment of another species, if it were not for the addi- 

 tional fact that Mr. Wachsmuth finds specimens agreeing exactly with Owen 

 and Shumard's species in the ornamentation of the body, and yet having only 

 six arms to each ray, or thirty in the entire series, instead of eight to each ray, 

 as in that under consideration. 



P. Wort/ieni, Hall, agrees with this in having eight arras to the raj', but they 

 are much stouter, and differ in being roughened by numerous small asperities, 

 while its second radial pieces are much smaller, and each supports on each 

 side above only two very short pieces between it and the first bifurcations 

 above, instead of four. Similar differences are also seen in the details of the 

 other divisions, while the surface ornamentation of the two species is entirely 

 different, and the base of the Wortheni is flat or broadly concave, instead of 

 being moderately convex with merely a central concavity. 



Locality and position. Upper beds of Burlington Limestone, Burlington, 

 Iowa. No. 218 of Mr. Wachsmuth's collection. 



Genus PROTASTER, Forbes. 

 Protaster? gregarius, M. and W. 



The disc of this species is circular in outline, slightly convex above, and 

 measures from 0-20 to 0-30 inch in diameter. In most cases it looks as if merely 

 covered by a smooth membranaceous integument. Some casts of its external 

 surface, however, seem to show traces of flat, nearly smooth, imbricating scales 

 above. The five arms are slender, flexible, and rather long in proportion to 

 breadth. In a specimen with a disc measuring 0-25 inch in breadth, the diaine- 

 ter of the arms near the disc is only 0-05 inch. None of the specimens show 

 the entire length of the arms, though some fragments of them were seen lying 

 detached in the matrix, about 0-55 inch in length, without being complete at 

 either end. From the breadth and gradual taper of these it would seem prob- 

 able that when entire they may have been 0-75 to 1 inch in length. Their 

 impressions in the matrix give no indications of a longitudinal furrow along 

 the under side, but show that there were about six pairs of arm pieces in a 

 length of 0-16 inch. These pieces appear to be nearly though not exactly 

 opposite, and each one provided below with a comparatively large, round, deep 

 pit or pore, near the middle of its anterior side. Along their lateral margins 

 there appear to be impressions in the matrix of very small spines (one to each, 

 arm piece), though if such they must have been extremely short. Impressions - 

 of the upper side of the slender arms show them to have been somewhat 

 rounded above, with the nearly square arm pieces slightly alternating. Some 

 of the impressions seem to show traces of central pores or pits, one at the mid- • 

 die of each pair of pieces, though in others no traces of these are visible. 



We have numerous specimens of this little species before us, but as they 

 are all merely in the condition of casts and moulds in a very fine, somewhat 

 granular matrix, they do not show the details of its structure very clearly. As 

 tar as its structure can be made out, however, it seems to agree well with the 

 general features of the genus Protaster, as illustrated by Prof. Hall in the 

 Twentieth Report of the Regents of the University of N..Y..on.therSUte Cabi-. 



1869. J 12 



