594 ECIIINODERMATA CRINOIDEA chap. 



arras. One genus, Holopus, in shallow water in the Caribbean 

 Sea. 



Fam. 5. Comatulidae. — Stem in the adult broken off, leaving 

 only a stump, the centro-dorsal, covered with cirri. Six genera. 

 Antedon ( = Comatula) has already been described ; many tropical 

 species have numerous arms and often side-plates and covering 

 plates. Actinomctra is distinguished by its excentric mouth, and 

 by the fact that the centro-dorsal is flat and has cirri only round 

 its edges ; Atelecrinus has an acorn-shaped centro-dorsal, and the 

 basals are externally visible ; Eudiocrinus differs from Antedon 

 only in having five arms ; FromacJiocrinus is a remarkable form, 

 having ten radii (this is a unique feature in Crinoidea) ; finally, 

 Thaumatocrinus has basals externally visible, large persistent 

 orals and interradial plates, and in addition a short free appendage 

 of several plates on the anal interradius. Antedon and Actino- 

 metra are almost world-wide. Six species of the first have been 

 recorded from British waters, of which the commonest is Antedon 

 rosacea ; four others are distinguished by having longer cirri, and 

 do not seem to be well defined ; but 

 A. eschrichtii, a northern form, is larger, 

 and is distinguished by having long 

 proximal pinnules. The other genera 

 are rare, and occur in deep water. 



When we turn to survey fossil Cri- 

 noidea, we are met with a bewildering 

 variety of forms ranging from the Lower 

 Cambrian to the present day. As already 

 mentioned, there is no agreement amongst 

 experts as to how they should be classified. 

 Bather makes the fundamental cleavage 

 cri7ius, one of the Came- depend on the posscssion of two whorls 



rata, broken opeu to sliow r. i , • .1 i /-n- ntr a\ Xf 



structure, atnb, Ambu- of plates in the basc (Fig. 274), or of 



lacral groove enclosed in o^ly Qj^e whorl. ThcSC tWO divisions he 



covering plates ; 5, basal ; ,, -^ i ■» ;r 



R\ R-, K\ the three Calls DiCYCLICA and MONOCYCLICA rc- 



zTtteiV^''''°^""'"' ^*^^^'' spectively. He admits that in many 

 forms allied to Dicyclica the infra-basals 

 have disappeared ; these he terms " pseudomonocyclic " forms, 

 and believes that he is able to discriminate them from true 

 Monocyclica. The present autlior is utterly unable to believe 

 that the Crinoidea diverged into two groups on what is a 



