CRINOIDEA. 303 



Cyathidium Steenstr., Cretaceous and Tertiary ; Cotylederma Quenst., 

 Lias. 



Holopus is quite unlike any other Crinoid and it is impossible to fix the 

 systematic position. * We do not even know the constitution of the calyx 

 or the position of the anus. If the calyx is composed of basals and radials 

 only, the genus ought to be placed with the Larviformia. 



CYSTIDEA AND BLASTOIDEA. 



The Cystids and Blastoids are entirely extinct and their fossils 

 have only been found in the Palaeozoic rocks. They differ so 

 much from living Echinoderms that it is by no means easy to 

 interpret their structural features or to assign them to their 

 proper systematic position. It has been customary with zoolo- 

 gists in recent years to associate them with the Crinoids in a 

 subphylum Pelmatozoa, the remaining Echinoderm classes being 

 united in a second subphylum which has been called Eleutherozoa 

 (Bell) or Echinozoa (P. H. Carpenter). We cannot think that 

 it is for the advantage of Zoology to adopt this classification. 

 In the first place the Crinoids are not sufficiently distinct from 

 other living Echinoderms to justify their assignment to a 

 separate group of the dignity of a subphylum. In the second 

 place our knowledge of Crinoid anatomy is detailed and com- 

 plete, and based upon a minute study of living forms, while 

 our knowledge of Cystids and Blastoids is vague and unsatis- 

 factory to an exasperating degree. To take the forms assigned 

 to the Cystidea alone, we cannot even be certain whether we 

 are dealing with a single class or whether the range of structure 

 met with would not more properly be distributed over several 

 classes equal in value to the other Echinoderm classes. To the 

 zoologist the great interest attaching to the study of Cystids 

 consists in obtaining an answer to these questions. Their 

 association with Crinoids seem to us to make it more difficult to 

 obtain an answer. It places us in an entirely false position with 

 regard to them, for it implies that we have a considerable know- 

 ledge of their anatomy and so may cause us, in the light of our 

 complete knowledge of Crinoids, to strain our interpretation of 

 difficult or doubtful Cystidean structures in a manner and to an 

 extent which may lead us far from the truth. For these reasons 



* It is placed by Jaekel and Bather near the Eugeniacrinidae on 

 account of its arm structure. 



