THE CYSTIDEA 69 



the corresponding broadening of 1. ant. B ; the sinking of the 6 KR, 

 and 3 iRR, so as to alternate with the 10 BB, except in posterior 

 IR, which is entirely occupied by anal plates ; the repeated bifurcation 

 of the arms, as in Caryocrinus, but probably to a greater extent, accom- 

 panied by an increase in number of tegminal and accessory plates, the 

 arrangement of which is unknown. Stereom-folds are visible. Thecal 

 plates nodose (cf. Hemicosmites), hence the name of H. armatus given to 

 the only specimen known. 



Throughout this family there is no strict correlation between the arms 

 and the cup-plates. There is in both structures a dominance of the 

 number three or six, it is true ; but each arm has not its own radial plate 

 supporting it, as in Crinoidea. Indeed, an arm may be borne by a plate 

 that, on all other grounds, would be considered as interradial. Moreover, 

 the trend of evolution in the family is parallel to the probable evolution 

 of early Camerata, rather than towards that or any other Crinoid type. 



ORDER 3. Aporita, Zittel (1879, restr.) 



Cystidea in which pentamerous symmetry affects the food- 

 grooves and thecal plates, probably also the nerves and ambulacral 

 vessels, but not the gonads. The food-grooves are exothecal and 

 circumoral. The stereom and stroma show no trace of folds, 

 rhombs, diplopores, or anything other than the finely porous 

 structure characteristic of all Echinoderm stereom. 



One may regard this order as a backwater in the stream of pro- 

 gress, derived perhaps from the Rhombiferi, but leading nowhere 

 in particular, and only retained because the forms referred to it 

 cannot be placed elsewhere. The arrangement of the thecal plates, 

 and a homoplastic resemblance to Hypocrinus (p. 178), have sug- 

 gested to some a connection with the Crinoidea. 



There is only one family, the CRYPTOCRINIDAE, and in it the thecal 

 plates are arranged in four circlets. Cryptocrinus, von Buch (1840 and 1 845), 

 Ordovician, Russia (Fig. XXXVII.). Theca small, irregularly spheroidal, 

 composed of four circlets of plates. Aboral circlet of three unequal plates, 

 produced by fusion of an original five, the unfused plate being in right 

 anterior interradius. Above these is a circlet of five rather large hexagonal 

 plates, following on and alternating with which are five smaller sub- 

 pentagonal plates. These surround an irregular pentagon in which are 

 the minute plates of the fourth circlet, and other small tegminal plates. 

 Five main food-grooves lead from the mouth to facets borne by these 

 adoral plates. The free brachioles rising from the facets must have been 

 slender. The anus, with valvular pyramid, lies between two plates of 

 the third circlet, either supported on a plate of the second circlet, or 

 separated therefrom by a small supplementary plate. The hydropore 

 appears to have been in the adoral plate opposite the anterior food-groove, 

 and left of the anus, which therefore occupies much the same position as 

 in Glyptocystidae. Another pore, perhaps excretory, lay in the adoral 



