310 PROCEEDINGS OF THE ACADEMY OF [1881. 



This genus is closely allied to Abacocrinus, but differs essen- 

 tially in the number of basals, and in the disposition of the anal 

 plates. It differs from Periechocrinus in the depressed form of 

 the body, in the I'obust nature of its plates, and in the construc- 

 tion of the vault. 



By former authors, and originally by Owen and Shnmard, Megisio- 

 crinus was referred subgenerically to Actinocrinus. It was 

 thereby sufficiently distinguished as long as the majority of all 

 Actinocrinidffi were referred to the genus Actinocrinus, but since 

 this has been subdivided and restricted to a limited group, it is 

 evident that Megistocrinus should form a distinct genus. Angelin 

 made it a synomym of Sagenocrinus but this has underbasals and 

 single-jointed branching arms. 



Generic Diagnosis. — Species generally large. Body depressed, 

 wider than high; calyx in form of a cup or basin, of which the 

 lower portions, up to the middle of the first radials, form the 

 bottom part ; plates heavy ; dome not much elevated ; symmetry 

 bilateral. 



Basals three, of equal size, spreading horizontally and producing 

 an equilateral, hexagonal disk, which is either on the same plane 

 with the surrounding radials and first anal plate, or, which forms 

 between those plates a sunken area without any further concavity. 

 Primary radials 3X5, almost of like form, slightly decreasing in 

 size upward ; all wider than high, and generally hexagonal. The 

 third radials have an obtuse angle, and support from their sloping 

 sides a single secondary radial, which is either axillary and suc- 

 ceeded by 1 X 2 X 10 tertiar}^ radials, or is followed by two rows 

 of plates which are arranged transversely, alternating with each 

 other and interlocking, and which, as in Abacocrinus, form the 

 main trunks of the arms. In species with tertiary radials these 

 support two rows of plates, similar to those of the preceding 

 order, and there are within the body three or four arm tranks 

 instead of two. 



Arms bifurcating throughout their length, and in a similar 

 manner as in the preceding genus. The arms, from the body up, 

 are composed of two rows of short joints, and are fringed with 

 long, round pinnules. 



Interradials numerous ; one in the first, two in the second, and 

 three in the third series, with several more ranges of minute 

 pieces above. First anal plate in line with the first radials, and 



