SYSTEMATIC PART. 155 



perisomic plates in the disk that suggested the Larviformia, and the sac- 

 Hke prolongation of its posterior area the Fistulata ; all Camerata have 

 a rigid disk, contrasting therein with the Articnlata in which the disk is 

 pliable. 



The condition of the ambulacra, whether resting upon the tegmen or 

 being incorporated into it by means of their covering pieces, or whether 

 constituting open furrows upon the disk, is of more than family importance ; 

 but the exposure of the covering pieces, and their concealment wholly or in 

 part by the encroaching perisome, are not even reliable generic characters. 



The presence or absence of orals, and their greater or less symmetry or 

 asymmetry, have very little classificatory value, except in the Larviformia, 

 in which they are the only plates of the disk. 



The condition of the anus, whether in form of a simple opening directly 

 piercing the calyx, or situated at the end of a tube, has been generally 

 regarded as of generic value. 



Of considerable importance is the presence or absence of pinnules, w^hich 

 is correlated with other characters by which very large families are dis- 

 tinguished, e.g.^ the Cyathocrinidse from the Poteriocrinidse ; and it is the 

 name-giving character of the Pinnata and Impinnata. 



The condition of the arms, their simplicity, their mode of branching, and 

 the arrangement of their plates — whether uniserial or biserial — afford 

 useful characters for distinguishing genera; but as all biserial arms are 

 derived phylogenetically, as they are embryologically, from the uniserial 

 ones, it must be expected that at a certain time both structures occur side 

 by side in the same genus. 



The construction of the stem, the form of the joints, the length of the 

 internodes, have comparatively little value in the classification of Palseozoic 

 Crinoids, and are of generic value only in rare cases. Perhaps if the stems 

 were oftener and more completely preserved, it would help in the identifica- 

 tion of species. 



With regard to species, their recognition is to a considerable extent the 

 expression of the individual opinion of the observer, and in large collections 

 it is often difficult to distinguish between species and variety. So long as we 

 have to deal with new forms, represented by unique specimens, the task is 

 simple enough. But when large numbers of specimens are brought together, 

 in different states of preservation, presenting different conditions of growth 

 and size, and exhibiting the various shades of individual variation, it is not so 



