AGELACRINID^:. 



253 



spond with the " ambulacral grooves " of the Star -fishes. 

 The opening of the mouth appears to be placed in the 

 centre of the five arms ; and in one of the spaces between 

 the arms is situated a little pyramid of from five to nine 

 calcareous plates, which forms the valvular " ovarian aper- 



The genus is wholly Silurian, 



ture" (fig. 145, A, o). 

 Devonian, and Carbonifer- 

 ous, and it has been gener- 

 ally placed under the order 

 Cystoidea. It differs, how- 

 ever, from these in its total 

 absence of a stalk of at- 

 tachment, and in the pos- 

 session of ambulacral pores, 

 perforating the test. On 

 the other hand, it recalls 

 the Asteroidea in the struc- 

 ture of the ambulacral 

 grooves ; while the some- 

 times articulated, some- 

 times imbricated inter- 

 radial ( interambulacral ) 

 plates recall to mind the 

 Perischoechinidce ; though 

 we know of no parasitic 

 or sessile examples of the 

 Star-fishes or Sea-urchins. 

 In the allied genus Ed- 

 rioaster (fig. 145, B) the 

 body is sessile, circular, 

 discoid, and convex, and 

 covered with irregular pol- 

 ygonal plates. The mouth 

 is large and somewhat pen- 

 tagonal. Eadiating from the mouth are five ambulacral 

 grooves, each formed of a double series of plates ("am- 

 bulacral ossicles"), with two pores between each pair of 

 plates. There are thus four rows of ambulacral tube-feet, 

 as shown by the pores, which penetrate the entire thickness 



B 



Fig. 145. A, Upper surface of Agelacrinus Cin- 

 cinnatiensis, enlarged two and a half diameters. 

 Lower Silurian (after Hall). B, Upper surface 

 of an imperfect specimen of Edrioaster Bigsbyi, 

 of the natural size. Lower Silurian (after Bil- 

 lings), o, Ovarian pyramid. 



