LOWER PALAEOZOIC FAUNAS 137 



Crinoidea, the only enduring class of Pelmatozoa, 

 occurred in the Cambrian, but the oldest types are not 

 well known. It is interesting to find that one Ordovician 

 genus, HybocrinuS) had definite, though elementary, 

 hydrospires, suggesting derivation of the class through, 

 or parallel with, early Blastoid types. Extreme regu- 

 larity in plating, and prolongation of the food-grooves 

 on to "arms/' are the dominant characteristics of the 

 class. The calyx (patina) may be built of two or three 

 cycles of plates ; it is difficult to believe that the two 

 characters (which imply contrasted arrangement of 

 nervous and other structures) can have been developed 

 more than once in Crinoid history. The two sub- 

 classes thus recognized, Monocyclica and Dicyclica, 

 followed nearly parallel and synchronous courses of 

 evolution, but Monocyclic forms are almost extinct 

 to-day, while modified Dicyclic families are profusely 

 represented in modern faunas. 



Monocyclica are divided into three orders, Inadunata, 

 Adunata and Camerata. The Inadunates, with calyces 

 built of basals and radials (with or without intercalated 

 anal plates), are known from the Cambrian, and still 

 survive. They are generally small forms, and rarely 

 prove abundant. Hybocrinus, from the Ordovician, has 

 been mentioned above; the Silurian Pisocrinus is the 

 only genus well known from British Lower Palaeozoic 

 horizons. Adunata, with calyces like those of the Ina- 

 dunata, but supporting massive tegmens, were mainly 

 of Upper Palaeozoic date ; but Hapalocrinus and Marsi- 

 pocrinus, from the Wenlock Limestone, prepared the 

 way for the abundant Platycrinidae of the Carboniferous 

 Camerata, in which the calyx contains many incorporated 

 arm-plates, were similar in stratigraphical range to the 

 Adunata ; Periechocrinus is a familiar Silurian forerunner 

 of the later Actinocrinidae. 



