188 NEW YORK STATE MUSEUM 



attached to any other body, and yet all that have been found 

 occur in clusters. The aboral surface is gently depressed, with 

 large marginal plates projecting downward as a peripheral ring. 

 On the oral surface the five ambulacra are extremely narrow, 

 directed radially, and when near the periphery bend abruptly 

 to the right (coutrasolar) and are extended like whiplashee 

 parallel to and very near the periphery, slightly undulating in 

 their coui*se. These narrow ambulacra are covered with very 

 fine interlocking, triangular, arching cover plates. The thecal 

 plates on both aboral and oral surfaces are imbricating or have 

 that appearance.^ 



This organism resembles A gel. kaskaskiensie, Echin. 

 sampsoni (Echinodiscus-Discocystis-Agela- 

 discus) and Lepidodiscus squamosusin the narrow, 

 whiplike character of its rays. It differs from the two former 

 (but not from Ivcpidodiscus) in the squamous aspect of the thecal 

 plates and from all in the uniform contrasolar direction of all 

 the rays. 



We can not hope to arrive at a proper or approximate generic 

 designation for this and related agelacrinites unless we under- 

 take here to consider briefly what constitutes the values of cer- 

 tain morphologic features in this group of species. 



Thecal plates. The earlier or Siluric agelacrinites have a 

 squamous surface of imbricating plates; A. a 1 1 e g a n i u s of the 

 Chemung and the lower Carbonic A. squamosus and A. 

 b 1 a i r i have the same. The middle Devonic A. h a m i 1 - 

 t n e n s i s has irregularly shaped mosaic thecal plates with 

 sculptured and ridged surfaces, while the Carbonic species of the 

 '' Echinodiscus " group haye smooth and regular polygonal 

 mosaic plates. 



In many species a circular wall bounding the apexes of the 

 rays is built of large and strong plates supported on the 

 periphery by much smaller plates (A. h a m i 1 1 o n e n s i a) or 



*It may be doubtful whether on the Interradil and aboral surfaces these plates always actually 

 overlap each other as they clearly do In the peripheral region. They have a scaly appearance which 

 may be due to the greater thickening at the Inner or proximal edges on both surfaces. 



