364 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



in all but two of the known genera in both families of three equal plates. 

 The orientation of these plates is uniform throughout both families, viz. : 

 the interbasal sutures are directed toward the anal plate and the two 

 antero-latei-al radials. The anal plate in the Batocrinidfe is heptagonal, 



and is followed by three plates in the first 



jy'"" ^^Q? interbrachial row, viz. : a second anal and two 



S^^O ^A*^ Rc^j interbrachials ; whereas in the Actinocrinidae 



rjjyCc] '^C~^ y^^Un ^'^ ^"'^' plate is hexagonal, and is followed 



'i^^-^ Mf^r^'^Y ^"'•^ by the two interbrachials, — there being 



■ ^ Cvwr'^A^^^^ "*^ second anal. We regard the presence of 



"^"^^V^^i) ^^^^P ^ second anal within the first row of inter- 



<jo 00 ^3 ^~^->i 9R? brachials as of considerable morphological im- 



"L/, %^ y portance. It is represented without exception 



~ ^oSd" from the Lower Silurian to the close of the 



Devonian in all monocyclic Camerata in which 



Fig. 18. Batocrinus. 



anal plates occur ; in the Kinderhook, for 



h = basals. B ^ radials. x = special anal 



plate, ix = suppiemeniary anal, ibr = the first time, wc find both forms together : 



the Actinocrinoid with but two plates above 

 the first anal, and the Batocrinoid with three. 



When there are three basals, the superior edge of the basal cup is nine- 

 angled, six of the angles being salient, the others retreating, and the suture 

 lines are directed to the middle of the two antero-lateral radials and the 

 anal plate. By this arrangement two of the radials are heptagonal, and the 

 three others hexagonal. In Compsocrinus with four basals, the posterior and 

 anterior basals are pentangular, the two others quadi'angular, the anterior 

 radial hexangular, all others heptangular. 



The alternate bifurcation of the arms from the two main branches of 

 the rays, which is characteristic of the ActinocrinidjE, forms another well- 

 marked distinction between the two families, and is also very constant. 

 Among the Batocrinidse this mode of bifurcation occurs only in Gennwo- 

 crinus from the Upper Devonian, but there it goes along with a second 

 anal ; and we must regard this genus as a transition form toward the 

 Actinocrinidse. The paired arm structure also, although not a constant 

 character, occurs only among the Batocrinidse ; never among the Actino- 

 crinidse. Another significant fact, tending to confirm the separation of the 

 two groups as distinct families, is that so far as known the Batocrinidse 

 disappear in Europe with the dawn of the Carboniferous ^- except perhaps 



