PLATYCEINID^. ygg 



depressed for the reception of the column. The interbasal suture lines 

 indeterminable; the basi-radial and interradial sutures on a level with 

 the plates. Eadials rapidly spreading, considerably wider than long; the 

 length of the lateral faces less than the width of the lower one; the'' limbs 

 incurving, sloping outward and forming wide and deep notche's between 

 the plates; their upper ends sharply angular and higher than the upper part 

 of the facet. Facets large, occupying half the width and fully one third 

 the length of the plates; deeply and broadly excavated at the upper end. 

 Costals broadly triangular, rapidly sloping at the middle, their lateral exten- 

 sions knife-like. First distichal placed obliquely, the second wider and 

 higher than the first, and overlapping it laterally so as to touch the costals. 

 Palmars of the same proportions as the distichals. Arms generally five to 

 the ray, exceptionally six ; cylindrical, very heavy and proportionally short. 

 The ventral disk of this species was high, as indicated from fragmentary 

 parts preserved in one of the specimens. The interambulacral regions appar- 

 ently consisted of five plates, of which the middle one of the first row is very 

 large, and nearly twice as wide as high, the two at the sides as long but 

 narrower; the former having a central spine. Column distinctly elhptic, 

 the edges of the joints studded with numerous small nodes. 



Horizon and Zoca%. — Lower Burlington limestone; Burlington, Iowa. 

 Ttjpes in the collection of Wachsmuth and Springer. 



Bemarks.-We were at first inclined to regard "this form as a variety of 

 P. TandelU; but the differences in the size of the basal disk and the propor- 

 tions of the arms, as well as in the form of the dorsal cup, are so remarkable 

 that we concluded eventually to treat it as a full species. It agrees in the 

 form of the dorsal cup with the Di.scoideus group, but in the ornamentation 

 and arm structure it is nearer P. TancMli, P. verrucosus, and P. hemisphericus, 

 and it doubtless represents a transition form. 



Platycrinus spinifer var. elongatus W. and Sp. (nov. var.). 

 Plate LXVII. Fig. 7. 



Very closely allied to P. spinifer; the dorsal cup deeper, but very little 

 expanding, the basal disk larger and almost flat. The radials rest upon the 

 inner edges of the basals, and are as long as wide ; their facets much wider 

 than high, directed obliquely upwards ; they extend to less than one third 

 the length of the plates, but occupy half their width. Costals trigonal 



