ORINOIDS. 155 



not lacking. The forms found in the Lower Carboniferous 

 present a somewhat different aspect from those of the earlier 

 periods, for they have the calyx very much shortened and pro- 

 portionately broadened at the base of the free arms, besides 

 differing in several other respects. 



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 

 Burlington, where it became extinct. Both genera appear to 

 have a larger number of interradials, especially on the anal 

 side, than any other of the Actinocrinoid groups. 



Amphoracrinus approaches Agaricocrinus in the flattened 

 dorsal cup, the high, often inflated ventral portions, and in the 

 shape and arrangement of the plates of the aboral side. The 

 anal side and the arms connect it with Actinocrinus and Per- 

 iechocrinus: 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 pieces, 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 Am- 

 phoracrinus has been referred to above. In the anal structure 

 it is identical with Dorycrinus, having the same arrangement 

 of plates, and a similar vertical, rounded ridge, near the top of 

 which is the simple anal opening. TJie arms are exceedingly 

 stout, somewhat like those in certain forms of Actinocrinus 

 from the lower part of the Burlington limestone, but very much 

 heavier. Agaricocrinus, Amphoracrinus and Dorycrinus prob- 

 ably began to diverge from the more typical members of the 

 family, and from each other, about the same time ; and this was 

 apparently during the middle or lower Devonian. In the upper 

 part of the Burlington or the early Keokuk a small group of 

 forms departed still farther. These have been placed under 

 Alloprosallocrinus, though it is doubtful whether the differences 



