NATURAL SCIENCES OF PHILADELPHIA. 165 



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 posteri- 

 orly.* 



Megistocrinus Uvamii, Owen and Shumard ; M. plenus, 31. crasstts, White ; and 

 M. brevicomis and 31. snperUtus, described by Prof. Hall under Acnnocri7ius.-\ 

 Also our M. parmrostris of this paper. All of which are from the Lower Car- 

 boniferous. 



? 2. Subgenus Saccocrinus, 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 sometimes with a pro- 

 boscis. 



Aclinocrinus {Megistocrinus) Whitei, Hall, and Act. {Saccocri7ius1) amplus, M. 

 and W. 



Megistocrinus parvieostris, M. and "W. 



Body rather small, of the usual short cup shape, about one-fourth wider than 

 high. Base nearly even with the surrounding first radial and first anal pieces. 

 First radials near one-third wider than lon^. Second radials a little smaller 

 than the first, hexagonal in form. Third radials as wide as the first, but 

 shorter, pentagonal or hexagonal, and each supporting on each superior slop- 

 ing side two brachial pieces in direct succession, upon the last of which rest iwo 

 or three series of double alternating pieces before the commencement of the free 

 arms, making two arm openings to each ray, or ten to the entire series. First 

 anal piece wider and a little shorter than the first radials, supporting three 

 smaller pieces in the next range, above which there are four or five in the 

 third, and five in the fourth ranges, which latter connect with numerous very 

 small pieces forming and surrounding the little short proboscidiform opening. 

 First interradials about as large as the second radials, and bearing two smaller 

 pieces in the second range, three in the third and two or three in the fourth, 

 with a few minute pieces in the latter. 



Vault a little convex, composed of moderate sized, convex or tuberculiform 

 pieces, the largest and most prominent of which is in the middle, and the others 

 surrounding it, while a few minute pieces are intercalated between and around 

 the latter, particularly on the anal side. Anal opening in a short, little pro- 

 boscidiform protuberance, placed entirely below the horizon of the arm bases, 

 and directed a little obliquely downward. Body plates, excepting the small 

 ones, connected with the opening, all rather tumid, and separated by exca- 

 vated sutures, somewhat indented at the corners of the plates. Arms and col- 

 umn unknown. 



Height of body to top of central node of the vault, 0-80 inch ; breadth, 0-96 

 inch ; height to arm openings, 0-58 inch ; do. to anal opening, 0-38 inch. 



• This diagnosis is not intended as a full description of the genus, but merely to give 

 the characters distinguishing the typical section of the genus. If such Devonian specsies 

 as M. I'llus, Hall, and Acliiiocrinus abnormis, Lyon, belong to this genus, they could not be 



Eroperly included in the typical section with the Carboniferous species. They have the 

 road depressed form of Mftjisloeriiius proper, but differ in having thin even body plates, 

 without impressed sutures, and the first, at least, in having a subcentral opening in the 

 vault, while the plates of the species u/»i'*r//u's are, when well preserved, ornamented, even 

 on the vault, with delicate radiating stria. From the appearance of its arm bases, in the 

 specimens we have seen, they would seem to have also been constructed differently from 

 those of the Carboniferous species, while the arm openings present the curious character 

 of being each divided by a transverse horizontal septum into two distinct openings, one 

 directly over the other. The anal opening in this species, however, is lateral, and much 

 like that of the typical Jfe5'i'.s-«f)cr(H;(s. In each of tlie four specimens of this species we 

 have seen, from "the original locality, there are only the usual five series of radials (three 

 each) instead of six, as in that from which Mr. Lyon's description was drawn up. 

 f Art. minor, Hall, is believed to be the same as his A. brevicomis. 



1869.] 



