250 The American Naturalist. [March, 
On the other hand, Megistocrinus, with its thick, heavy plates, 
boldly sculptured, and having a very depressed calyx, reached its 
greatest development in the middle Devonian. It continued, 
though in greatly lessened numbers, to the upper part of the Bur- 
lington, where it became extinct. Both genera appear to have a 
larger number of dorsal interradials, especially on the anal side, 
than any other of the Actinocrinoid genera. 
Amphoracrinus approaches Agaricocrinus in the flattened dor- 
sal cup, the high, often inflated, ventral portions, and in the shape 
and arrangement of the plates of the aboral side. The anal 
side and arms connect it with Actinocrinus and Periechocrinus: 
with the former by the possession of usually only two ossicles in 
the second tier, by the absence of the marked vertical row of anal 
plates, and by the presence of a short sub-central anal tube; with 
the latter by the peculiar structure of the free arms. 
Agaricocrinus is remarkable for the greatly depressed form of 
the calyx, the dorsal cup being nearly flat, or, as in some of the 
later species, decidedly concave. Its resemblance to Amphora- 
-crinus has been referred to above. In anal structure it is identi- 
cal with Dorycrinus, having the same arrangement of plates, and 
a similar vertical rounded ridge near the top of which is the 
simple anal opening. The arms are exceedingly stout, somewhat 
like those in certain forms of Actinocrinus from the lower part 
of the Burlington limestone, but very much heavier. Agarico- 
crinus, Amphoracrinus, and Dorycrinus probably began to 
diverge from the more typical members of the family and from 
each other about the same time, and this apparently took place 
during the middle or lower Devonian. In the upper part of the 
Burlington or early Keokuk a small group of forms departed 
somewhat from the typical species. These have been placed 
under Alloprosallocrinus, though it is doubtful whether the 
differences are great enough to render a separate, generic term 
useful. The chief point of distinction is the position of the anal 
opening, which is placed at the end of a short ventral tube, 
instead of being a simple aperture in the test, as in Agaricocrinus. 
It seems, however, that much more importance has been hitherto 
