136 NEW YORK STATE MUSEUM 



Spencer and Schuchert that P. narrawayi presents an apical 

 instead of an oral aspect. 



In U. pulchella, the inner ends of all floor plates meet the 

 median lines of the rays and are closely joined to those of the 

 opposite column. The first pair in each ray is greatly increased 

 in size and forms a very definite radial jaw, the proximal inner 

 edges of the plates projecting into the oral cavity. The following 

 floor plates (with the exception perhaps of the second and third 

 pairs) are thickened apically to a remarkable degree and retain 

 impressions of the surfaces of the radial s and supramarginals 

 against which they developed. The floor plates are set with their 

 inner edges a little nearer the mouth and form a very thick floor, 

 flat on the oral side and strongly convex on the apical. 



In P. narrawayi the inner ends of the first three pairs of 

 floor plates can not reach those of the opposite column, but they 

 run along the curved, apical, inner border of the axillary infra- 

 marginals and become interradial in position. If we assume that 

 for every cover plate there was a floor plate, then the first floor 

 plates form the " secondary jaw "of this species (see my former 

 papers for quoted terms), the second floor plates form the inter- 

 radial pairs of " suboral epineurals " and the third floor plates are 

 those I formerly designated as " second epineurals." One may 

 pass around the inner apical margins of the great axillary infra- 

 marginals and find no break in the series of floor plates. Though 

 the first members have moved toward the oral surface, the second 

 members have joined each other on the interradius and cover 

 the first pair apically. The axillary inframarglnal is excavated 

 on its Inner, apical surface to receive the divorced second pair of 

 floor plates. This condition Is shown In Hudson 1913, figure i. 

 The floor plates of the arm are not thickened vertically. Those 

 preserved in one arm have their Inner ends nearer the mouth; in 

 another arm they are tipped away from the mouth. The floor Is 

 built like a long and steeply angled roof with the ridge placed 

 apically. There was no marked vertical thickening of the plates, 

 no evidence for an arm lacking viscera, no convex apical side 

 of floor with Impressions of the radlals and Inframarglnals, though 

 some deformity may be due to this cause. Orally the floor surface 

 seems to have lacked the transverse grooves and other complex 

 features found in Urasterella. Unlike the latter genus, the double 

 series of floor plates formed the weakest elements In the structure 

 of the arm. 



