68 The Ohio Journal of Science [Vol. XIX, No. 1, 



In A . cincinnatiensis is found perhaps the greatest differences 

 between the lateral and median rows of covering plates. Plate 

 II, Figure 8, and Plate VIII, Figure 51A and 51B. The outer 

 covering plates are grooved on their sides and the cylindrical 

 median plate fills the space left by the grooving of two adjacent 

 lateral plates. Its free end only is visible, alternating with the 

 opposite median plate and both entirely covered from the sides 

 by the two rows of lateral covering plates. 



A weathered part of the tip end of an arm of A. cincin^iati- 

 ensis, Plate II, Figure 11, gives the clue to these different 

 types of arrangements of the cover plates. Only one half of 

 the ray is left. At the distal end of the ray the plates are 

 side by side in one series as in ^4. pileus. 



As one looks proximally he finds every other plate dropping 

 downward and toward the food groove. This is approximately 

 the present condition in A . aiistini. Still nearer the mouth the 

 alternate arrangement, a large lateral row of cover plates and a 

 small median row is the normal condition that characterizes 

 A. cincinnatiensis. 



A young specimen of A. cijicinnatiensis , 6 mm. 'in diameter, 

 from the Corryville at Mason, Ohio, shows the primitive con- 

 dition of cover plates. (Figure 29A, Plate V). The food 

 grooves are but slightly bent. In one ray there are six plates 

 on each side of the groove and as they have, as yet, not been 

 crowded in the course of their development the plates are even 

 and side by side as in A. rectiradiatiis and A. pileus. It is 

 possible that the next to the first plate at the base of the ray 

 No. 1 is dropping inward and will take position as one of the 

 median cover plates, producing the double row on each side, 

 which characterizes A. cinci?inaHensis. If so, the growing 

 point of the food grooves is at the proximal end of the ray by 

 intercalation of plates rather than at the distal end by addition 

 of plates. 



This crowding together of the cover plates is correlated with 

 an increase in length of the food grooves. Again A . rectiradiatus 

 furnishes the primitive straight stages. A. aiistini, Plate III, 

 Figure 13 and .14, A. pileus, Plate I, Figure 3 and 4, A. cincin- 

 natiensis, Plate I, Figure 2, Plate II, Figures 8 and 11, and most 

 all A. holbrooki, Plate V, Figure 26, all have rays which are 

 longer than the direct distance to the peripheral rim plates. 

 The rays of A. austini at their outer ends turn slightly at the 



