A MONOGRAPH OF THE EXISTING CRINOIDS 63 



(6) Notocrinida. Deep radial pits. Articular facets occupying almost the 

 whole of the synarthrial face. 



(1) Side and covering plates moderate. Brachials and pinnules rounded. 

 Genital glands ui the arms. Notocrinidae. 



(2) Side and covering plates well developed. Brachials and pinnule seg- 

 ments prismatic — triangular in cross section. Genital glands in the pinnules. 

 Asterometridae, new family (including Asterometra and Pterometra) . 



(4) Macrophreata. Centrodorsal conical, hemispherical, or, rarely, discoidal. 

 Centrodorsal cavity large and deep. Basal star generally reduced (Atelecrinidae 

 with large basals). Muscular fossae on the articidar faces of the radials very large, 

 equal to or larger than the rest of the face (relatively smallest in the Antedoninae). 

 No calcareous plug. Synarthrial backward projection present. Syzygies numerous, 

 with few septa (except in the largest forms). Arms 10 with very rare exceptions. 

 Side and covermg plates strongly reduced (largest in the HeUometrinae). Proximal 

 pinnides strongly polymorphous and differentiated. 



(a) Centrodorsal more or less hemispherical (rarely discoidal as in Eumetra 

 and other genera). Centrodorsal cavity rather moderate. Cirri in alternating rows. 

 Synarthrial backward projection moderate. Articular facets of the synarthries 

 occupy almost the whole of the synarthrial face. Antedoninae, Perometrinae, Thy- 

 sanometrinae, and perhaps also Thaumatocrinus. 



(h) Centrodorsal conical to columnar. Centrodorsal cavity excessively large. 

 Cirri in columns, or tending to be arranged in columns. Synarthrial backward 

 projections generally very large. Articular facets narrow, occupying a part of the 

 synarthrial face only. Zenometrinae, Bathymetriiiae, HeUometrinae, fComatonia, 

 Isometrinae, Atelecrinidae, Pentametrocrinidae (perhaps excluding Thaumatocrinus). 



This rearrangement of the comatidid types was based so far as possible upon 

 characters which can be determined in fossil species, especially, therefore, characters 

 which are evident in the centrodorsal and in the radials. Altogether too httle weight 

 is given to the characters furnished by the arms and pinnules. 



Structurally the pinnules are the most distinctive organs of the comatulids and 

 the organs showing the widest departure from the conditions in the other crinoid 

 types. Among the comatidids the pinnules serve as food-gathering organs, as append- 

 ages contauiing the genital organs, as locomotor organs used for crawling or collec- 

 tively as a web or vane in swimming, as tendrillike grasping organs supplementing the 

 cirri, as pectinate cleaning organs, as sensitive tactile organs, and as spinelike organs 

 of defense. When serving for purposes other than the collection of food they usually 

 lack ambulacral structures. In some cases the enlarged pro.ximal segments of the 

 oral pinnules are embedded in the dorsal perisome and form part of the body wall, 

 as in many other crinoids. 



Since the extraordinary diversification of the pinnules is the chief distinguishing 

 feature of the comatulids as compared with other crinoids, it is in the pinnules that 

 we find the most abundant and the most definite clues to their true interrelationships. 

 The structure of the centrodorsal and of the radial pentagon is of the greatest value 

 in showing the more fundamental lines of departure from the generalized type, and 



