210 



THE EDRIOASTEROIDEA 



with axial folds, and consisting of : BB (5 ?, sutures not clear) ; RR, 5, 

 alternating with BB, and receiving the distal ends of the ambulacra ; 

 interambulacrals, one large one and an uncertain number of smaller ones, 

 in each interradius ; 5 slightly-pitted plates of spear-head shape, stretching 

 up between the ambulacra to the oral pole and simulating of Crinoidea 

 or A of Blastoidea, but perhaps being only proximal covering-plates. 

 The anus pierces one interambulacrum, and slightly disturbs the 

 pentamerous symmetry of the theca. From the mouth 5 ambulacra 

 stretch about half-way down the theca ; the adambulacrals (side- or 

 flooring-plates) appear almost anchylosed, but the pores between them 

 are very clear, and one can trace the original median line of suture ; 

 the ambulacrals or covering-plates were stout, at least in the proximal 



-amb 



amb 



"As 



St 



Fia. VII. 



SteganoUastus ottawaenste, slightly restored from the type-specimens, and x 3 diaiu. 1, 

 oral surface ; 2, from 1. post, radius, adamb, adambulacrals or side-plates ; amb, ambulacrals 

 or covering-plates, mostly removed ; As, anus surrounded by small plates ; B, basal ; IR, large- 

 median interradial ; 0, 5 orals or proximal ambulacrals; p, pores between side -plates; 11, 

 radial ; St, fragment of stem. 



regions, where they seem to have combined with the spear-head plates to 

 form a solid roof over mouth and food-grooves. Stem small, round, with 

 lumen less than half the diameter. Fifty years ago SteganoUastus would 

 have been described as a generalised or synthetic type, with Cystid, 

 Blastoid, Crinoid, and Asteroid affinities ; it is simpler to regard it 

 as a specialised Edrioasteroid, in which features common in stalked 

 genera of other classes have been evolved independently under similar 

 conditions of existence. 



Cyclocystoides, Billings and Salter (1858), Ordovician of N. America. 

 and Britain (Fig. VIIL), probably belongs to this class, though not to any 

 of the recognised families. It is hardly well enough known to make 

 the type of an order as yet. A ring of stout ossicles, more regular than 

 that seen in Agelacrinus and Edrioaster, forms a frame between which are 



