251 
and functional relations of undoubted importance ; in most 
instances, ignorance of the anatomy and physiology of 
extinct organisms prevents any definite indication of luhat 
value such characters may really be, in a classificatory point 
of view. 
The fragmentary condition in which fossils are often 
found preserved, and the frequently limited supply' of 
material, as well as total uncertainty about the functions 
dependent on obscure peculiarities of structure, too often 
limit the number of characters available for classification. 
On the other hand, the continuous increase which has been 
made of late years to False ontological knowledge has 
rendered the solution of many problems possible, with which 
formerly we were unable to cope. Perhaps few better 
examples could be named than the subjects of the present 
communication. 
The four Groups of Crinoids herein described all agree 
in the number and general arrangement of the plates which 
form the calyx or body-cup ; in other words, the numerical 
elements of the essential portions of their generic formulae 
are identical. The special proportions of these plates, how- 
ever, and the general characters of the crinoids, differ in each ; 
and when the whole sum of the resemblances and differences 
of genus with genus is compared, and when the types are 
examined in their entirety, few will dispute the propriety of 
establishing the Groups as above indicated. 
In Potenocrinus and Dactijlocrinus, the form of calyx and 
the mode of attachment to the column, are similar ; but Dacty- 
locrimis differs from Poteriocrinus in having only a single 
series of very elongate free radials, a character suggestive of 
the genus Scap/iiocrinus, from which Dactylocrinus is clearly 
separated by the contour of the calyx, the more elongate plates, 
and the much longer ossiculrc of the arms, which arc not 
wedge-shaped, but have their articulating margins parallel. 
i8 
