1890.] ; Genesis of the Actinocrinide. ° ~ 245 
rounded by a greater or less number of smaller pieces. The 
anal aperture may be a simple opening immediately back of the 
orals, or at the end of a long ventral tube. 
The fundamental modifications in the arrangement of the 
various plates give trustworthy criteria for the basis of genera; 
while the ornamentation, relative size and shape of the calyx 
ossicles form very satisfactory features for the distinction of 
species. The taxonomic values attached by different paleontolo- 
gists to the various characters are not the same. This difference 
of interpretation, however, appears to arise largely from inatten- 
tion to the ontogenetic history of the living forms of the class. 
But this diversity of opinion, happily, is rapidly lessening, with 
the prospect of a speedy agreement, at least in the main features, 
as to the relative worth of the separate structures in classification. 
General Morphological Changes.— Before passing, however, 
to morphological details, it may be well to call attention to some 
widely-spread variations recorded. Briefly summing up, then, 
the statements recently’ made in a general consideration of the 
most marked anatomical features displayed by the Carboniferous 
-crinoids of the Mississippi basin, it may be said that these 
organisms, from the beginning of the lower carboniferous to the 
close of the Keokuk, showed: (1) a wonderful and extremely 
varied development of different structural characters; (2) a con- 
stant increase in size and massiveness of test; (3) a peculiar _ 
change in ornamentation, which, from the delicate style of the 
earlier forms, gradually grew more and more bold and rugged ; 
and (4) many curious modifications in minor particulars. 
These striking and wide-spread phenomena point to very 
decided changes in surroundings, such as might have resulted 
from a gradual decrease in the depth of the sea, a slight diminu- 
tion, in the density of the water and the introduction of fine sedi- 
ment in consequence of the nearer proximity to the drainage 
courses of the young continent, or marked alterations in the 
coastal contour of the neighboring mainland. There probably 
were acting also numerous other though less apparent influences. 
Indeed, these suggestions find substantiation in the stratigraphy 
1 Keyes : Carb. Echin. Mississippi Basin, Am. Jour. Sci., Sept., 1889. 
