DISCOVERY OF THE DISK OF ONYCHOCRINUS 483 



Calpiocrlnus having twenty main arm branches or trunks, about 

 equal, each pair bearing ramules inside the dichotom; while Homalo- 

 crinus has ten such main branches. The first, or lowest, ramule is 

 usually the largest, and may sometimes branch again. Variations 

 in this respect give rise to intermediate forms, and while well-marked 

 specimens of the two genera are strikingly distinct, it is easy to see 

 how the two arm characters shade into one another. In C. fimbriatus 

 (Plate VII, Fig. i) it will be seen that the ray bifurcates into two 

 equal divisions, and each of these divides again into two main branches 

 or trunks of about equal size, from the inner side of each of which 

 are given ofT three or more lateral ramules. In H. parabasalis 

 (Plate VII, Fig. 9) and H. dudleyensis (Fig. 12) the first bifurcation 

 in the ray produces two main arm branches having similar ramules 

 of which the lower one is the largest. If this lower ramule increases 

 further, until it approaches the size of the outer branch, and also 

 begins to give off in turn subordinate ramules, then we shall have 

 the condition, approximately, of Fig. i. Thus we may expect to 

 find intermediate stages of this progression; and when the lower 

 ramule of the ten-branched form becomes considerably enlarged, we 

 are in doubt whether upon this character to call it Homalocrinus 

 plus, or Calpiocrlnus minus. And then comes the old question: 

 Shall we throw them all into one genus because of the connecting 

 links, or maintain the genera upon their typical forms ? Of course, 

 the correlation of the radianal, if present, will avoid this difficulty; 

 but I find this to be quite unequally developed in the English forms 

 of Homalocrinus, and it is probably so in the Swedish — being so 

 small in the type specimen of H. parabasalis. In the former some 

 of my specimens have it only half the size of the radial, some about 

 equal to it; and in some, not otherwise distinguishable, I cannot see 

 it at all. I think the difference in this respect may be partly due to 

 the unequal development of the infrabasals, which may sometimes 

 conceal the radianal, which in this form is located wholly within the 

 ring of basals, as in Sagenocrinus. This may also be considered as 

 an interesting case showing how, under the influence of an extrava- 

 gant modification, characters otherwise important were greatly over- 

 shadowed, and a close and rapid transition occurred, tending to 

 produce an intermingling of such characters. 



Phillips' genus Euryocrinus, which has been heretofore considered 



