THE CRINOIDEA 



95 



all modifications, which are numerous and vastly divergent, these 

 elements persist. A circlet of radially situate infrabasals may 

 also be present. Below basals or infrabasals there follows a stem, 

 which, however, may be atrophied or totally lost. 



Although many Rhombifera simulate Crinoidea in the penta- 

 merism of the theca or the possession of exothecal extensions of 

 the food-grooves, yet in none are those extensions supported by 

 plates that are clearly outgrowths from the abactinal system of 

 thecal plates ; in none is there the intimate correlation between 

 brachia and radialia that obtains in the Crinoidea. This class 

 therefore cannot be derived from the Rhombifera, as many 

 structures might otherwise lead us to suppose ; the presence of 

 brachia also forms a clear distinction between it and Diploporita 

 and Blastoidea. A further difficulty in tracing the origin of the 

 Crinoidea is furnished by the occurrence of perfectly developed 



DO 



Fio. I. 



Analysis of the cup and brachial elements of Hybocystis problematicus. The outlines of the 

 food-grooves (vg) are dotted. 



species in the Lower Cambrian, while representatives of all crinoid 

 orders are plentiful in Ordovician rocks. Further research, how- 

 ever, may throw back the origins of other Echinoderm classes ; in 

 any case, negative evidence when Cambrian rocks are concerned 

 counts for little. 



Certain features in some of the older Crinoidea seem to throw 

 light on their ancestry. Such are the presence of hydrospires, 

 comparable to those of Cadaster, in Carabocrinus (p. 172) and 

 Hybocrinus (p. 145); the presence in these and other genera of 

 well-developed deltoids (A), over the edges of which pass the ambu- 

 lacra, while the posterior A frequently shows signs of a bydropore 

 (Fig. XXXVI.) ; the absence of a brachium from certain radials in 

 Baerocrinus (Fig. LVII. 4) ; the greater development of three radials 

 in many Inadunata Monocyclica (see p. 144). We are thus led to 

 a form not unlike that which is actually presented by Hybocystis, 

 Wetherby (1880), from the Ordovician of Kentucky (Fig. I.). 

 This has 5 large subequal basals, 5 radials, and 5 deltoids. The 



