1898] WACHSMUTH AND SPRINGER ON CRINOIDS 389 



the arms and over the tegmen to the mouth. The axial nerve-cord 

 is dorsal in position, i.e., it lies at the very bottom of the ventral 

 groove, and may be shut off from it by an ingrowth of stereom, so 

 as to lie in a distant 'axial canal.' Tlie food-groove lies on the 

 ventral surface ; it is fringed by extensions from the ambulacral or 

 water- vascular system, and is protected by small plates that can 

 shut down over it, and are called ' covering-plates ' or ' ambulacrals.' 

 The stem or column consists of a series of ossicles, which may be 

 circular, pentagonal, stellate, or oval in section, but always are 

 pierced by a central ' axial canal,' which contains a prolongation 

 from each of the five chambers of the dorsal nerve-centre. 



The modifications of this simple type are numerous and in very 

 different directions. Some of them are extraordinary, and structures 

 have been produced that long remained unsolved enigmas. More- 

 over, many modifications, both of ordinary and extraordinary type, 

 can be shown to have occurred more than once in groups of very 

 diverse origin ; for instance, the work under review describes a 

 Carboniferous Platycrinid, CamiJtocrinus, in which the stem has 

 undergone the same remarkable curvature and arrangement of cirri 

 as affected the Silurian Heterocrinid, Herpetocrhuis. Were all the 

 crinoids that ever lived placed before us, with no indication as to 

 their distribution in time or space, it could only be by an unlikely 

 chance that anyone would hit on their true relationships ; and to the 

 classification of one author, that of another could always be opposed. 

 Fortunatel}^ we do know something of the succession of this remark- 

 able series of forms, and we can trace, with some assurance of 

 correctness, a few lines of descent which it would be hard to 

 controvert. The whole history of the attempts to classify the 

 Crinoidea, shows the gradual recognition of these principles, and the 

 gradual emancipation from the older habit of lumping forms together 

 because they were alike in structure without considering how the 

 likeness arose. This has been the history of all branches of 

 systematic zoology, and if the latest classification of the Crinoidea 

 does not attain perfection, it is partly because there are still too 

 many gaps in our knowledge of the geological history of the class, a 

 point admirably emphasised by Messrs Wachsmuth and Springer, 

 partly because it is one thing to have a conviction, and another thing 

 to have the courage to act upon it, and by so acting to overturn 

 established and accepted beliefs. 



For the purpose of this article it is necessary to consider only 

 those modifications of the simple crinoid type that influence the 

 classification, in other words, those modifications w^hich are believed 

 to indicate some affinity between the forms exhibiting them. 



In the simple type the arms are freely moveable on the radials, 

 they are distinct from the cup, i.e., do not help to enclose the viscera, 



