NATURAL SCIENCES OF PHILADELPHIA. 163 



Locality and position. Lower beds of the Burlington group. Lower Car- 

 boniferous. No. 58 of Mr. Wachsmulli's collection. 



Genus MEGISTOCRIXUS, 0. and S., 1850. 



The type upon which this genus was founded {M. Evansii, 0. and S.""*) has a 

 short, broad, cup shaped body, with a depressed vault, and sides moderately 

 expanded above, and rounded under below to the flat anchylosed base, which 

 is usually a little impressed, or less prominent than the first radial and first 

 anal pieces extending horizontally outward all around it so as to form a part 

 of the under side. In some species the base is not properly impressed, though 

 it can rarely be said to project be3'ond the surrounding next range of pieces. 

 The body plates are moderately thiciv, and separated by well defined, or rather 

 deep sutures, so as to present a more or less convex surface, without sculp- 

 turing or radiating costifi, though there are rarely small indentations at the 

 corners of some of the plates. 



The nearly or quite flat vault is composed of unequal, irregular, more or less 

 tumid or convex pieces, of moderate size, the middle one sometimes rising into 

 a prominent, rather pointed node, that may be, in some cases, even developed 

 into a short spine. The opening is decidedly lateral, often penetrating the 

 anal side beloio the horizon of the arm bases; sometimes it is on the same ho- 

 rizon as the arm openings, or rarely slightly above them. It is never situated 

 in a thickened protuberance, however, as in Dori/crinus and Agaricocrinus, but 

 always shows thin, broken, abruptly- projecting edges, as if, when entire, it had 

 been produced into a short, slender tube, or so-called proboscis, projecting out 

 horizontally backward. 



In the number and arrangement of the pieces composing the walls of the 

 body, up to the third radial pieces inclusive, this genus presents no essential 

 differences from Actinocrinus, with which it also agrees in having the arm bases 

 more or less grouped, or separated by interradial and anal spaces, and never 

 forming a continuous series all around, as in £atocrinus,f nor an ex])anded 

 disc, as in Strotocrinus. It not only differs from Actinocrinus proper, however, 

 in general physiognomy and the nature and position of the opening, but par- 

 ticularly in having its arms each composed of a double series of alternating 

 pieces beloiu all the bifurcations, as in Amphoracriiius, from which, however, it 

 differs widely in other respects. This peculiarity of having the arms each 

 composed of a double series of alternating pieces below as well as above the 

 bifurcations, is not only continued down to the body, but in some species each 

 division of the rays included as a part of the walls 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 the}' can be readily distinguished at a glance from the allied genera 

 such as Amphoracrimis, Agaricocrinus, Dorycrinus^ Civlocrinus, Strotocrinvs, &c. 

 There is, however, at least one, and probably two, known Carboniferous spe- 

 cies, standing as it were between Megistocrinus and Saccocrinus, and combining 

 the characters of both to such an extent that one of them (Act. (3Ipgist.) 

 Whitei, Hall) was referred by Prof. Hall to Megistocrinus (which he seems to 

 regard as a section or subgenus of Aclinocrinns), while the name of the other 

 was written by us, Actinocrinus [Saccocrinus ?) amplus,\ because we were con- 



* Owen's Geol. Report, Wisconsin, Iowa and Minnesota, pi. V, A, fig. 3. 



t We cannot believe that those remarkable truncated forms, with arm bases in contact 

 all around, and an erect subcentral proboscis, such as M. spinnsus, of Lyon (Proe. Philad. 

 Acad. N. S. Deer. 1861, pi. iv, fig. 7), really belong to Megistocrinus. 



X It is possible, as already intimated, that this may not be distinct from Prof. Hall's spe- 

 cies Wtiilei ; but as it is larger and more robust, however, and has its body plates more con- 

 vex, and without the ridge seen extending up the radial series of the species \Vliite>\ whicti 

 also differs in some other details, and 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 



1869.] 



