164 PROCEEDINGS OF THE ACADEMY OF 



vinced that it is not a true Actinocrinus, and believed it related to Saccocrinus. 

 From direct comparisons with Prof. Hall's t.ypical specimens of the species M. 

 Whitei, in the Museum of the University of Michigan, Professors Winchell and 

 Marcy were also led to refer Silurian species of Saccocrhnis to Megistocrinus. 



The Silurian typical forms of Saccocriuus have the same arm structure, as 

 well as essentially the same arraug-ement of body plates, as ^legistocrinus, 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 subcentral opening (or proboscis?) instead of a 

 decidedly lateral proboscidiform opening in the vault, ^lius presenting a deci- 

 dedly different general physiognomy from the typical Carboniferous forms of 

 Megistocrinus. If we had only the typical forms of these two groups to deal 

 with, there would be no difficulty iu separating theiu. The two species, or 

 varieties, Whitei audi ampins, however, are not so easily disposed of, since they 

 have the same thin smooth plates, without impressed sutures, seen in Saccocri- 

 nus, 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 

 the}' also equally agree in the number and arrangement of the body plates. 

 We have never seen a specimen 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 

 specimen figured by Dr. Roemer iu his work on the Silurian fossils of Tennes- 

 see, shows the vault to be protuberant in the middle and provided M'ith an 

 opening there, with some a])pearance 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 Whitei, as already stated, there is a 

 small subcentral opening in the depressed vault, without any traces of a pro- 

 boscis, and we can now scarcely doubt that this is the case with the amplus, 

 and the vault of both these species also differs from that of the typical Megis- 

 tocrinus 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 Saccocrinus should be 

 ranged as a subgenus under Megistocrinus. At any rate, if the species Whiiei 

 and amplus are to be included under Megistocrinus at all, we think they should 

 certainly be at least placed in a separate subgenus from the typical forms, and un- 

 til these questions can be more definitely settled from the study of more extensive 

 collections we prefer to retain for this subgenus the name Saccocrinus. With 

 these limits the genus Megistocrinus would include the following American Car- 

 boniferous forms : 

 1. Megistocrinus, Owen and Shumard, 1850. 



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



to these two horizons, we are not entirely convinced that they are identical. All the spe- 

 cimens of the species, or variety, amplus we have seen are in a more or less crushed con- 

 dition, which in some instances caused the vault to protrude in such a way as to lead us 

 tu believe it provided with a central or subcentral 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 Mrgistocrinus. Mr. Waehsmuth informs us. how- 

 ever, that he has recently found a specimen of the species Wliilei showing that it has a 

 small subcentral simple opening, much as in Strolocrinus, section (a), and from the close 

 relations of our amplus to that species, it is highly probable that it also has a similar open- 

 ing, without a proboscis. 



* It is proper that we should also state here that there are considerable differences of 

 form among the Silurian spe<aes o{ Saccocrinus; for instance, we have now before us, from 

 the Niagara beds of Chicago, rude natural casts, having all the characters apparently of 

 undescribed species of this group, that are as short and broad as even the typical Carbo- 

 niferous forms of il/c(;(stocn«i«s; wliile specimens of Saccocrinus Christi/i. Hall, now before 

 us, from the Niagara beds of Waldron, Indiana, have the body presenting almost exactly 

 the form of the typical specimens of the Carboniferous species ^\'liHei, with which, through 

 the politeness of Prof. Winchell, we have been able to compare them. 



[July, 



