1890.] Genesis of the Actinocrinide. 243 
GENESIS OF THE ACTINOCRINID. 
BY CHARLES R. KEYES. 
rm crinoids belonging to the family Actinocrinide reached 
their greatest development and expansion during the lower - 
Carboniferous period. In the American rocks the variety and 
number of these forms is indeed remarkable, perhaps nowhere 
equaled in any other age or region. 
As regards the distribution of the group in time and space, 
and the phylogenetic history of the camarate forms in general, 
many pregnant suggestions have recently been offered by certain 
terranes in the Mississippi valley. But until quite lately attention 
has been turned in other directions than towards the solving of 
these problems; to the purely geologic considerations, and to 
the description of species. That such a large number of specific 
and higher terms should have been proposed, many of which 
are now.regarded as invalid, is not surprising when it is remem- 
bered that serial comparisons were made only in exceptional 
cases, and that most of the forms show great variation, even now 
‘often offering more or less difficulty in definitely limiting the 
several sections. More recently efforts have been turned towards 
the structural features of the different groups, with most gratify- 
ing results towards a better understanding of the class. For a 
long time previous the lack of a sufficient variety and quantity 
of well-preserved examples of the occurring species had greatly 
embarrassed all efforts of this kind; and in many cases had led 
to very erroneous conclusions concerning the real structure of | 
various parts of the skeletal arrangement. Late finds, however, 
have in great measure removed many of the difficulties that 
earlier would have very much hindered any attempts towards a 
satisfactory solution of the two questions already alluded to, 
They have also supplied enough additional data to render profit- 
able the consideration anew of the entire stratigraphy of the 
Carboniferous rocks of the broad continental interior, with the 
