FOSSILS OF THE BUELIXGTON GROUP. 395 



ber and arrangement of the body plates. We have never seen a speci- 

 men of any of the Silurian species of Saccocrinus showing the vault, but 

 all the casts we have examined seem to show that it was nearly flat, 

 and had either a subcentral opening or proboscis, and no traces of a 

 decidedly lateral opening, as in typical forms of Megistocrinus. A speci- 

 men figured by Dr. RCEMER, in his work on the Silurian fossils of Ten- 

 nessee, shows the vault to be protuberant in the middle, and provided 

 with an opening there, with some appearance of being surrounded by 

 the remains of the base of a proboscis, though it may possibly be only 

 a simple opening in a prominence. In the Carboniferous species White!, 

 as already stated, there is a small subcentral opening in the depressed 

 vault, without any traces of a proboscis, and we can now scarcely doubt 

 that this is the case with the ampins, and the vault of both these species 

 also differs from that of the typical Megistocrinus in being composed of 

 innumerable minute pieces. 



Although there are a few points in regard to the relations between 

 these groups that we have not yet been able entirely to clear up, we are, 

 from all the facts now known to us, inclined to believe that Saccocrinw 

 should be ranged as a subgenus under Megistocrinus. At any rate, if 

 the species White! and ampins are to be included under Megistocrinns 

 at all, we think they should certainly be at least placed in a separate 

 subgenus from the typical forms ; and until these questions can be 

 more definitely settled, from the study of more extensive collections, we 

 prefer to retain for this subgenus the name Saccoerinus. With these 

 limits, the genus Megistocrinns would include the following American 

 Carboniferous forms : 



1. MEGISTOCBINUS, Owen and Shumard, 1850. 



Body short, broad, and composed of rather thick convex pieces ; base 

 flat, impressed, or scarcely more prominent than the first anal and first 

 radial pieces ; vault composed of moderate sized convex pieces ; opening 

 decidedly lateral, produced in the form of a small, short proboscis ! and 

 directed posteriorly. 



Megistocrinns Evansii, Owen and Shumard, M. plenum, M. crassus, 

 White ; and M. brevicornis and If. snperlatus, described by Prof. HALL 

 under Actinocrinus. Also our M. parcirostris of this paper. All of 

 which are from the Lower Carboniferous. 



f 2. Subgenus SACCOCRESUS, Hall, 1852. 



Body usually more elongated, with a protuberant base ; body plates 

 thin, even, or not convex ; vault composed of small or minute pieces, 

 and provided with a small subcentral simple opening, or possibly some- 

 times with a proboscis. 



Act inner inn* ( Meywtocrinus ) Whitei, Hall, and Act. ( Saccocrinus f } 

 M. and W. 



