594 ECHINODERMATA CRINOIDEA chap. 



arms. One genus, JTolopus, 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. Actinometra 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; Promacliocrinus 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 

 ■g experts as to how they should be classified. 



Fig. 273.— Calyx of Actino- Bather makes the fundamental cleavage 

 crinus, oue of the Came- depend On the possession of two whorls 



rata, broken open to show „ , ^ . ,, , ,„• ^^, ,s. » 



structure, ami, Ambu- ot platcs lu the basc (Fig. 274), or of 

 lacrai groove enclosed in only one whorl. Thcsc two divisions he 



covering plates ; B, basal ; 



R\ Ji-, R", the three Calls DiCYCLIGA and MONOCYCLICA re- 



ZUtelV^'""^'"""' ''^'''' 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 author is utterly unable to believe 

 that the Crinoidea diverged into two groups on what is a 



