ON SOME PELMATOZOA FROM CHAZY LIMESTONE OF NEW YORK Id 



lire c on the same plate shows rather clearly a series of growth 

 lines, or rather rest lines, where periodic pauses seem to have 

 been made in addition to the edge of the plate. 



The resemblance of these plates to the basals of certain Rho- 

 docrinidae (especially to those genera with concave bases and 

 two or more interradial plates over the no longer truncate upper 

 edge of the radial) lies in their approximate size, the bent con- 

 dition, the thinner proximal portion, the radiating ridges on the 

 distal surface, the depressed apex, the occasional visceral ridge 

 [see text fig. 8] and the more than usual number of angles. In 

 the Rhodocrinidae, however, the distal ornamented area is usually 

 much greater than the bent proximal area. The plates once recog- 

 nized are easily separated from all others. 



Bibrachials. In each radius of the third circlet is a pair of 

 usually hexagonal plates each about one and one third times as 

 long as the greater width of the radials and a little less than 

 half as wide as they are long. The bibrachials meet over the 

 apex of a radial in one long, straight suture and their narrow 

 (sometimes rather pointed) distal ends reach the boundary of 

 the oral surface and together support the end plates of an ambu- 

 lacrum and usually four very short brachioles. The outer edge 

 of each plate has three sides ; the two lower meet the end plates 

 of the two rows of interbrachials and the remaining side meets 

 the horizontal outer third of the base of one of the great deltoids 

 and thus also reaches the oral boundary. 



The outside of the plate is ornamented with transverse, fold- 

 like, rather rough ridges which become less prominent and dis- 

 appear as they reach the long common suture of the two plates. 

 The inside surface is smooth. The face of the common suture 

 is very smooth and near the middle of the plate this suture 

 occupies half its width, the plates together making a very strong 

 element of the theca. The outer suture is crossed by numerous 

 grooves which, at least on the upper half, mark the position of 

 the pores or slits on the deltoid through which water passes 

 from the hydrospires to the exterior. These features may be seen 

 in figures 1, m and n, on plate 4. The number of external, trans- 

 verse ridges and the number of grooves across the outer suture 

 will depend on the age of the plate as we shall see under the 

 description of the deltoids. 



These plates have been called bibrachials without any inten- 

 tion of signifying that they are homologous with the brachials 

 of crinoids. They support the distal, not the proximal end of 



