162 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



The least departure from the structure of the Larviformia toward the 

 Fistulata is found in the genus Carahocriniis, in which the tegmen is com- 

 posed of five asymmetrical orals, meeting laterally and by their inner 

 ends, four of them resting against the radials, the posterior one being 

 separated from them by a number of irregular perisomic pieces, which 

 enclose a short anal pyramid. Somewhat higher differentiated is the 

 tegmen of Cyathocrinus alutaceus Angelin — C. ramosus Bather — (Plate III. 

 Fig. 6), whose ambulacra are subtegminal, but the orals are separated 

 from the radials by a narrow belt of perisome 5 contrary to other species of 

 Cyathocrinus^ in which the ambulacra rest upon the tegmen, and the orals 

 are, or seem to be, in a state of resorption. In Hyhocrmus, the ventral sac is 

 as small as in Carahoeriniis, and it has large orals resting against the radials ; 

 but the lateral edges of the plates are covered by the SaumpldUclien. The 

 Cyathocrinidse have a large ventral sac, and in the tegmen a madreporite, 

 which was probably unrepresented in the Poteriocrinidee, in which the sac 

 itself is perforated. The ventral sac made its appearance in the Hybo- 

 crinid^, Carabocrinid^, and Anomalocrinidse as a very insignificant protuber- 

 ance ; in the Heterocrinidae, Belemnocrinid^, and especially in the Cyatho- 

 crinidae, and Poteriocrinidae, it attained enormous dimensions, but dwindled 

 down in the Encrinid^ to almost nothing, although some of them still have 

 well defined anal plates. We do not restrict the Encrinid^ to forms without 

 anal plates, such as Encrinus, Stemmatocrinus, and Erisocrinus ; but include 

 among them the genera Eupachycrinus, Cromyocrinus, Ceriocrinus, and Oncocrinus, 

 in which the anal area passes through all possible transition stages. We 

 make the reduced size of the sac, and the highly differentiated articulation 

 between the radials and brachials, the distinctive characters of the family. 



The Camerata constitute a compact and well limited natural group, and 

 they are a highly speciahzed type, which by extraordinary development 

 reached a stage of extreme differentiation, and produced a ventral structure 

 apparently so different from that of other groups, that it was for a long 

 time found impossible to homologize its plates with those of the other 

 Crinoids. They represent a type of rapid culmination and development, 

 possessing already in the earliest known forms well defined pinnules, and the 

 biserial arm structure in most of their families being permanently established 

 at the close of the Silurian. The organization of the Camerata may not be 

 intrinsically higher than that of the other groups, but they very clearly 

 represent a higher state of development than Haplocrims or Symhathocrinus, 



