118 Dr. F. A. Bather — Studies in Edrioasteroidea. 



Specimen E 15900 from Kirkfield is probably from Horizon 2, 

 which is well developed at that place. 



I have no external evidence as to the horizon of the specimens from 

 near Belleville, or of that from Peterborough. 



^. biffsbf/i has also been recorded from the Fusispira and Nemato'pora 

 beds, in the middle of the Trenton group, in Minnesota (N. H. 

 Winchell & E. 0. Ulrich, Final Kep. Geol. Surv. Minnesota, vol. iii, 

 pt. ii, p. cxxiii, 1897). 



General DESCErpiroN. 

 The theca has an approximately circular periphery, and a diameter 

 varying between 20 and 50 mm. As in Binocystis barroisi (Study I) 

 and Edriomter buchianus (Study II), the general shape is that of 

 a Tam-o'-Shanter cap or a Breton beret (Text-fig. 1). 



floor ing 



coi^ering ^,^^^^^^''^'\j, 

 plates JL-^mM^^:^.' nwu.th 



inleT- , 



TodicLL 



plcdes / 



anus -^^^^^^^M^^^wuter-pore 

 "pores in 

 food-groove 



Fig. 1. Edrioaster bigsbyi. The adoral face, sHghtly restored from specimen A. 

 The cover-plates are removed from rays I, IV, and V, but remain on 

 rays II and III. Natural size. 



The mouth is at or near the centre of the convex face, and from 

 it five subvective grooves pass over the theca on to the concave face. 

 Each groove is sharply curved as it approaches the periphery ; the 

 curve of the right posterior is probably always solar (= dextral), 

 that of the others is contrasolar in the type-species. Each groove 

 is floored by an alternating series of plates (floor-plates), and roofed 

 by alternating movable cover-plates, one to each floor-plate. The 

 median line of each groove is depressed to form a channel, from which 

 branches are given ofi to right and left between the floor-plates. 

 Each branch ends at a pore, which passes down the suture between 

 the adjacent floor-plates into the thecal cavity. These pores are close 

 to the margins of the groove, but beneath the cover-plates. 



The mouth opening (peristome) is surrounded by skeletal elements 

 continuous with the floor-plates, and is roofed by plates continuous 

 Avith the cover-plates. 



The thecal plates of the convex face between the rays (interradials 

 or interambulacrals) are relatively large, irregular in shape and size, 

 and do not imbricate. 



In the posterior interradius, two large interradials, adjoining the 

 peristome, are traversed by an elongate hydropore. 



