Summit Structure of Pcntremites. 503 



Shumard, previous to the discovery of the arms of both 

 species. It dillcrs in the greater proportional height of the 

 calyx, the character and arrangement of the nodes, and 

 the mode of bifurcation of the arms. 



Locality and position, in the lower division of the Bur- 

 lington Limestone, liurlington, Iowa. 



M// oivn Cabinet and that of Mr. Charles Wachsmuth. 



Platycrinus incompkis (n. s.) Body cup-shaped, grad- 

 ually expanding from the base, which is rather broad, 

 more or less depressed in the centre, and considerably bent 

 up at the sides ; suture-lines much depressed, those be- 

 tween the base and first radials sometimes so much so as 

 to give the base the appearance of being protruded ; arm- 

 facets sub-semicircular, occupying about one half the 

 width of the first radials ; first radials higher than wide, 

 full in the central parts ; second radials very short, some of 

 them not quite filling the facet in the first radials ; first 

 division of the rays consisting of two plates each, abutting 

 against each other by their inner ends ; upon the second of 

 these, the second divisions take place, which are composed 

 of the same number of plates as the first, and abut to- 

 gether in the same manner; other divisions take place, 

 with the same number of plates intervening ; as near as 

 can be ascertained, seven or eight arms to each ray. The 

 arms beyond the last bifurcation are rather short and 

 small, but between the body and the last bifurcation, they 

 are quite robust. The dome is rather high, covered with 

 tumid plates, small, except five at the centre, which are 

 larger, and arranged with their longest diameters directed 

 to the anal, and each interradial space, all meeting together 

 at the summit ; the one over the anal space is the largest, 

 the anal aperture being situated about half way between 

 it and the top of the calyx. Surface without ornamen- 

 tation. 



The calyx of this species somewhat resembles, and was 



