MESOZOIC FAUNAS 179 



Cretaceous two presumably Hydrozoan genera appear 

 in considerable profusion. Parkeria is a common fossil 

 of the Cambridge Greensand, and Porosphaera is 

 familiar (and zonally valuable) to all workers in the 

 Upper Chalk. Both of these genera are considered 

 to be nearly allied to Hydr actinia > but they have been 

 referred by some observers to the Porifera or even to 

 the Foraminifera. 



(E) ECHINODERMATA 



Although some forms of Crinoidea are locally abun- 

 dant, and fragments of Stelleroids occur in most marine 

 deposits, Echinoids are the predominant Mesozoic 

 Echinodermata. The rapidity of specialization and 

 wealth of variety, shown by that class during the era, 

 are in accord with its exuberant neanic elaboration in 

 the Upper Palaeozoic. So numerous (and withal beauti- 

 ful) are fossil Sea-Urchins in the Oolites and Chalk, 

 that the utter absence of Cystids, Edrioasteroids, Blas- 

 toids, and Adunate and Camerate Crinoids is insufficient 

 to recjuce the interest and importance of the phylum. 



Inadunate Monocyclic Crinoids, the sole surviving 

 order of the subclass, are fully represented ; the most 

 noteworthy type is Saccocoma, best known from the 

 Solenhofen Stone, but found in the British Kimmer- 

 idgian. TK15 1 small " feather-star " was one of the very 

 few Monocyclic types to assume " eleutherozoic " habits 

 by atrophy of the stem ; the quality was developed, 

 and is maintained, in the majority of Dicyclic Flexibilia. 



Dendrocrinoid Dicyclic Inadunata are especially 

 abundant in Mesozoic deposits. Encrinus^ the type of 

 the whole class, occurs as a rock-former in the Triassic 

 Muschelkalk. Pentacrinus^ a side-line of the persistent 

 , is a familiar fossil of the Lower Lias ; 



