210 PALEONTOLOGY OF ILLINOIS. 



we understand that genus, in having the vault very frequently armed with one 

 or more strong, sometimes greatly elongated spines. 



Of all the types included as subgenera, under Actinocrinus, by those who give 

 the widest limits to that genus, the Amphoracrinus group is most nearly allied 

 to Agaricocrinus and Megistocrinus, which we exclude from Actinocrinus, and 

 regard as most probably forming two separate and distinct genera from each 

 other, as well as from that under consideration. That they both, however, 

 (particularly the former) so closely approach Amphoracrinus, through a few 

 intermediate forms, as to be sometimes with difficulty distinguished, is no less 

 true than that very nearly all the known species can be readily separated into 

 these three groups. The chief distinctions between Agaricocrinus and Amphora- 

 crinus consist in the more depressed form of th« body and the concavity of 

 the under side of the former, by which its arm-bases are brought down upon a 

 horizon with the lowest part of the body. The arms in this group are also 

 stouter, and if we mistake not, invariably consist of only two to each ray; while 

 in Amphoracrinus there are often three in each of the posterior rays, and some- 

 times three or four to each ray all around. Again, we know of no species of 

 Agaricocrinus that has the vault provided with elongated spines, as we often 

 see in the Amphoracrinus group. The transition from one to the other of these 

 types is through such forms as Actinocrinus planobasalis, A. brevis, A. corniculus 

 and Agaricocrinus pentagonus, Hall. 



The Megistocrinus group seems to us to preserve its integrity as a genus more 

 decidedly than Agaricocrinus, although not differing always so widely, in general 

 appearance, from certain species of Amphoracrinus as the typical forms of 

 Agaricocrinus do. Still, even these forms, approaching most nearly Amphora- 

 crinus, differ in their more broadly rounded under side* and more depressed 

 summit, while their opening never (as in Amphoracrinus and Agaricocrinus) 

 penetrates a thickened protuberance, but in all the known species presents the ap- 

 pearance of an abruptly projecting j p7 , o6oscis. We have never seen a specimen 

 with this proboscis entire, or indeed retaining more than its base; but this base 

 always shows thin, prominent, fractured edges, and in the typical Carboniferous 

 species, at least, is situated lower on the side than the opening of Amphoracri- 

 nus or Agaricocrinus, being upon a line with, or even sometimes a little below, 

 the horizon of the arm-bases. Another difference, that may seem of little 



* We doubt the propriety of including in Megistocrinus those remarkable Devonian 

 species, with the under side broadly and abruptly truncated, and the vault provided 

 with a stout, nearly central proboscis, described by Mr. Lyon under the names M. 

 Knappii and M. spinosulus. Nor can we agree with our friends Prof. Winchell and 

 Prof. Marcy in referring to this genus the elongated, obconic, Upper Silurian species 

 described by them under the names M. Marcouanus and M. infelix, which belong to Sac- 

 cocrinus, Hall. 



