34 



This species is essentially different in its structure and general 

 form from all other twenty- armed species, and uo comparison with 

 any of them would tend to throw any light upon it. It is a 

 species, however, that has been frequently mistaken for B. roiundus, 

 just as B. oblatus and all other smooth species of Baiocrinus 

 have been. B. rohindns, as originally defined and illustrated, is 

 a twenty-one-armed species. It is much smaller, as a general rule, 

 than this species and never attains the size of the specimens 

 illustrated, in figure 28, though it has one more arm, in the radial 

 series. -It never agrees in form with this species, nor does it 

 possess as large a probocis. It is not constricted around the first 

 primary radials, as this species is, nor does it possess the same 

 form and number of plates in the azygou^ and regular interradial 

 areas. The resemblance between the two species is more fanciful 

 than real. The error probably arose iu this way, Meek & Worthen, 

 in describing B. dodecadaclyliis, (Geo. Sur. 111. Vol. 2, p. 207), 

 say: "Knowing that the number of arms sometimes varies to some 

 extent, in different specimen of the same species of criuoids, we 

 were at first inclined to think the form under consideration might 

 be only a young specimens of A. rotundus, but on comparing it 

 carefully with specimens of that species of the same size, we find 

 they possess the usual number of arms (20) in all our specimens, 

 and uniformly present the other differences mentioned." In writ- 

 ing the description of B. dodecadaciylus, Meek did not have a 

 single specimen of B. rotundus before him, but he had numerous 

 specimens of this species, some of them as small as the species 

 he was describing, and he had mistaken them for B. rotundtis. In 

 other publications, we find B. rotundus mentioned as twenty- armed 

 species, and like other errors, when put in circulation it continues, 

 because all authors do not take the time to correct it. Yandell 

 & Shumard described and illustrated iu the Geo. Sur. of Mo. a 

 twenty-one-armed species under the name of Actinocrinvs 7-i>tundus, 

 now known as Batocrinus rotundus, which is a species very rare, 

 in comparison with this species, at Burlington and the Illinois 

 localities and when this species was confounded with it, the species 

 was supposed to be very common, because two were included 

 under one name. The statement, tliat "the number of arms some- 

 times varies to some extent, in different specimens of the same 

 species of crinoids," we think is quite a mistake, if it refers to 



