CYSTOIDEA. 



453 



small calcareous 



imbricating manner 



plates, which may or may 

 and exhibits in its centre 



not overlap in an 

 the opening of the 

 mouth, protected by four " oral " plates. Radiating from the mouth 



es, ar 



of the anus (o), protected by a 



to 



spaces is situated the opening 

 " pyramid " of calcareous plates. 

 The species of Agelacrinus are 

 found in the Ordovician, Silu- 

 rian, Devonian, and Carbonifer- 

 ous rocks. The genus Edrio- 

 aster (fig. 323, b), from the Or- 

 dovician rocks, agrees in most 

 of its general characters with 

 Agelacrinus, but the ambulac- 

 ral grooves are furnished with 

 a double row of pores on each 

 side. 



Order 2. Diploporitid.e. 

 — In the Cystideans which are 

 included in this group, the 

 pores are linked or united in 

 pairs, and the individual calyx- 

 plates are pierced by several of 

 such pairs (fig. 321, b). There 

 is, as before pointed out, much 

 probability in the view put 

 forth by Loven, that the yoked 

 pores of the Diploporitidce 

 served for the passage of tube- 

 feet, and that they thus differ 

 in their real nature from the 

 " pore-rhombs " of the Rhom- 

 biferous Cystoids. If this view 

 be correct, the present group is 

 very sharply separated from both 

 the other groups of Cystoids. 



As the type of the Diplopo- 

 ritidce, the Ordovician genus Glyptosphcerites may be taken, in which 

 the globular calyx (fig. 319) is composed of numerous polygonal 

 plates, each of which is perforated by a larger or smaller number of 

 conjugate pores. The mouth is central, and is covered by five oral 

 plates, while converging to it are five branching ambulacral grooves, 

 at the terminations of which are the articular facets to which the 

 armlets or pinnulse were attached. On one side is the compara- 

 tively large opening of the anus, and between the two is a small 



Fig. 323. — a, Upper surface of Agelacrinus Cin- 

 cinnatiensis, enlarged two and a half diameters. 

 Ordovician (after Hall), b, Upper surface of an 

 imperfect specimen of Edrioastcr Bigsbyi, of the 

 natural size. Ordovician (after Billings), o, Anal 

 aperture. 



