394 PALEONTOLOGY OF ILLINOIS. 



each division of the rays included as a part of the Avails of the body has 

 the same structure nearly one-fourth of the way down the side, to within 

 one or two pieces of the third primary radials. 



The six or seven known true typical species of this genus form so 

 natural a group that they can be readily distinguished at a glance from 

 the allied genera: such as Amphoracrinus, Agaricocrinus, DorycrlnuH, 

 Catlocrinus, Strotocrinus, etc. There is, however, at least one, and prob- 

 ably two known Carboniferous species, standing, as it were, between 

 Megistocrinus and Saccocrinus, and combining the characters of both to 

 such an extent that one of them (Act. (Megist.^ Whitci, Hall,) was referred 

 by Prof. HALL to Megistocrinus (which he seems to regard as a section 

 or subgenus of ActinocrimisJ, while the name of the other was written 

 by us Actinocrinus ( Saccocrinus fj antplus* because we were convinced 

 that it is not a true Actinocrinus, and believed it related to Saecocrinn*. 

 From direct comparisons with Prof. HALL'S typical specimens of the 

 species M. Whitei, in the Museum of the University of Michigan, Pro- 

 fessors WINCHELL and MARCY were also led to refer Silurian species 

 of Saccocrinus to Megistocrinus. 



The Silurian typical forms of Saccocrinus have the same arm struc- 

 ture, as well as essentially the same arrangement of body plates, as 

 Megistocrinus, but differ in having a much more elongated, narrow 

 body, composed of thin, even plates (without excavated sutures) and a 

 protuberant obconic, instead of a flat or impressed base; also a subcen- 

 tral opening (or proboscis?) instead of a decidedly lateral proboscidi- 

 form opening in the vault; thus presenting a decidedly different gen- 

 eral physiognomy from the typical Carboniferous forms of Megitstocri- 

 nus. If we had only the typical forms of these two groups to deal with, 

 there would be no difficulty in separating them. The two species or 

 varieties, Whitei and amplus, however, are not so easily disposed of, 

 since they have the same thin, smooth plates, without impressed sutures, 

 seen in Saccocrinus, and nearly as protuberant a base, while their body 

 is exactly intermediate in form, and their arm structure the same as 

 in both of these groups, with which they also equally agree in the num- 



* It is possible, as already intimated, that this may not be distinct from Prof. HALL'S species Whitei, 

 but as it is larger and more robust, however, aud'has its body plates more convex, and without the 

 ridge seen extending up the radial series of the species Wldtei, which also differs in some other details. 

 aud came from the upper part of the Burlington beds, and ours from the lower, while scarcely any 

 species of Crinoids are believed to be common to these two horizons, we are not entirely convinced 

 that they are identical. All the specimens of the species or variety amplus we have seen are in a 

 more or less crushed condition, which in some instances caused tho vault to protrude in such a way 

 as to lead us to believe it provided with a central or siibcential proboscis, especially as several of the 

 specimens clearly show that it certainly has no traces of a lateral opening anywhere near the anal 

 side, as in the typical forms of Megigtocnnus. Mr. WACIISMITH informs us, however, that he has 

 recently found a specimen of the species Whitei showing that it has a small subcentral simple opening 

 much as in Strotocrinus, section (a), and from the close relations of our amplus to that species, it is 

 highly probable that it also has a similar opening, without a proboscis. 



