266 



THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



Remarks. — The Melocrinidae and Eucalyptocrinidse are the only mono- 

 cyclic families of the Camerata which have a pentagonal base, and in which 

 the radials are in contact all around. The latter, however, depart from the 

 Melocrinidas so widely in their ventral structure that there is no need of 

 further comparison. The Melocrinidae in many respects also seem closely 

 related to the Actinocrinidae, which followed them in time. Indeed the 



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CTCIIITD 



Fig. 12. Melocrims. Fig. 13, Stereocmius. 



(For the explanation of the letters, see Fig. 11.) 



superficial resemblance between the two families is often quite marked, but 

 a Melocrinoid is always readily distinguished from an Actinocrinoid by the 

 absence of an anal plate between the radials, and the consequent form of its 

 base, which is pentagonal. The Melocrinidae are among the earliest known 

 forms of the Camerata, being already found to the extent of six species in 

 the Trenton group;, contemporaneously with the earliest Rhodocrinidae and 

 Reteocrinidae. They increased in number in the Upper Silurian, but became 

 extinct before the close of the Devonian, before the appearance of the Actino- 

 crinidae, so far as existing collections show. The family includes fifteen gen- 

 era, of which ninety-five species are known ; fifty-four from America, and 

 forty-one from Europe. 



Some of the genera referred to this family have a perfectly pentamerous 

 dorsal cup, the posterior interradius being identical with the other four \ but 

 in others there are one or more anal plates interposed between the inter- 

 brachials, by which the pentamerous symmetry is disturbed. If this differ- 

 ence were well marked and constant, it would afford a basis for separation, 

 founded on the more or less complete absence of anal structures from the 

 dorsal cup ; but this being not the case, we arranged the genera only into 



