96 THE CRINOIDEA 



anus lies between the posterior A and the radial circlet, being 

 separated from the latter by a special anal plate (a;). The right 

 posterior radial is transversely bisected ; its upper smaller portion 

 (Rs) being pushed a little to the right by x. The striking 

 peculiarity of this form is the continuation of the food-grooves 

 over the thecal plates, as in Diploporita and Blastoidea. In the 

 right and left antero-lateral rays these pass over the edges of the 

 deltoids, over the radials, on to the underlying basals. In the 

 anterior and the right and left posterior rays there are two ossicles, 

 each as high as wide, supported on the summits of the radials ; 

 the grooves pass between the deltoids, over these ossicles, down on 

 to the outer surfaces of the radials. These ossicles form exothecal, 

 jointed outgrowths of the abactinal thecal plates ; a deep notch on 

 their inner surfaces, leading into the cup by a hole between the 

 deltoids, suggests that they bore, besides the ambulacral structures, 

 also extensions of the abactinal nerve-system. Therefore, although 

 incipient, they constitute true brachia, such as are found in no 

 Echinoderma except Crinoidea, and they show us the probable way 

 in which brachia originated. Hydrospires have not been described; 

 but, considering their occurrence in the closely allied Hybocrinus 

 (Fig. XXXVI.), they are likely to be found along the radio-deltoid 

 sutures, as in Cadaster. Brachioles fringing the grooves do not 

 seem to have been present, nor has a lancet-plate been observed. 

 These facts, as well as the five basals, prove that Hybocystis is not 

 an offshoot from Eublastoidea, as indeed its geological age forbids ; 

 but it may well be derived from early forms of Protoblastoidea. If 

 Hybocystis be admitted as actually ancestral, then the development 

 of brachia in only three rays sheds light on corresponding irregu- 

 larities of development in many simple and ancient crinoids, connect- 

 ing them in this respect with primitive Cystidea (see pp. 11, 53). 

 Another form suggestive of the connection of the Crinoidea with 

 the Blastoidea is Stephanocrinus, Conrad (1842), Silurian of N. 

 America and England. C. F. Roemer (1851), Joh. Miiller (1853), 

 and Pictet (1857), regarded it as a cystid ; Etheridge & Carpenter 

 (1883) and S. A. Miller as a blastoid; Dujardin & Hupe (1862) 

 and Hall (1851) as a crinoid ; Zittel (1879) as doubtfully a 

 blastoid. Wachsmuth & Springer (1886) proved the presence of 

 brachia, which make it unquestionably a crinoid, but said, "It 

 agrees by its oral and anal pyramid with certain forms of the 

 Cystids, while in its general habitus and in the position of the 

 ambulacra it agrees with the Blastoids." Stephanocrinus (Fig. II.) 

 has 3BB, 5RR, and 5A, arranged as in Eublastoidea, especially 

 Codasteridae. The radial processes are often prolonged into spear- 

 like spines (S), one in each interradius. Each ambulacral groove lies 

 in a deep sinus between the deltoids and radial processes, and it is 

 continued on to an arm, which rises from a single brachial at the end 



