228 



PALEONTOLOGY. 



above. In encrinus, it is composed of different sized circular 

 plates. And in pentacrinus, it is pentagonal. The body (figs. 

 159 and 161) resembles the corolla of a flower supported on a 

 stem. It is composed of the cupula or calyx, formed of nume- 

 rous plates united by their margins. To its upper surface 

 the five primary rays are attached. They present a great 

 variety of forms between the simple pinnate arms of apio- 

 crinus and the complicated branched rays of pentacrinus 

 there is almost every grade of development.* 



The arrangement of the plates in the structure of the 

 body affords good characters for generic division. The parts 

 now described do not exist in all crinoideans ; thus, coma- 

 tula has neither stem nor root, and in marsupites, (fig. 161) 

 the cupula is in the form of a purse, composed of one cen- 

 tral and three ranges of large polygonal plates. Each range 

 having five plates, the central plate has no articulating sur- 

 face for a stem, and the superior plates support the five arms. 



This figure of marsupites ornatus, 

 (fig. 161) from the Sussex chalk, 

 exhibits one of those extinct crinoi- 

 dea which formed the connecting 

 link between this order and the 

 genus euryale, one of the asterida. 

 The astrocrinida, echinocrinida, 

 and the cystidre possess a stem, 

 but are destitute of arms ; whilst 

 in the typical genera these parts 

 are all present. 



The number of pieces required 

 to compose the skeleton of the 

 true crinoidea is very great ; thus, 

 Parkinson calculated that there 

 were 26,000 separate elements in 

 the skeleton of encrinus monili- 

 formis ; and Buckland estimates 

 that there are above 150,000 ele- 

 FIG. lei. The marsupite. nients in that of pentacrinus 

 briareus of the lias. The palaeon- 

 tological history of the echinodermata presents us with some 



* Pict. Atlas ; beautiful figures of this interesting order are contained in 

 plates xlvi. to lii. ; see pi. xlvii. for anatomical details. 



