CLASSIFICATION 



117 



lent of the Kaskaskia into the Lower Coal Measures, where it has a character- 

 istic representative in Ampliicrinus; in this, however, there is exhibited a 

 tendency to reversion to the structure of the weak anal side. 



This group may be subdivided into three families, the first two of which are 

 more sharply differentiated from the third than they are from each other. The 

 character upon which the principal distinction is based is found in the condition 

 of the infrabasals, and its importance may be made clear by reference to the 

 growth of the calyx plates in the young comatulids. Leaving out of consid- 

 eration the infrabasals in the latter as usually too rudimentary for comparison, 

 the calyx cup in the early stages consists of basals and radials, both of which 

 form an important part of its wall ; the basals are erect, and help to enclose the 

 visceral mass. This is well illustrated in the young Antedon as worked out by 

 W. B. Carpenter, and in the larval Comactinia herein shown on Plates B and C. 

 The basals at first are not very far from parallel with the dorso-ventral axis 

 of the animal. At a later stage the basals undergo considerable diminution in 

 their external size, and become more and more recumbent in position until they 

 have slipped inward under the central part of the dorsal cup, disappearing from 

 sight altogether; at this stage they cease to take any essential part in the' struc- 

 ture of the calyx wall, and in some cases lose their identity by fusing to form 

 the so-called rosette. Primitively the infrabasals underwent a similar trans- 

 formation, and in the comatulids have been quite lost. This is more fully 

 explained by Mr. Austin H. Clark (Proc. U. S. National Museum, vol. 38, 

 pp. 117, 118), from whose language I have borrowed freely; and he finds that 

 among the Recent crinoids the best characters for the differentiation of the 

 higher groups are found in the stem and in the proportionate recumbency of 

 the basals. 



10 



Figs. 9-1 1 



modifications of infrabasals 



11 



9. Lecanocrinus; IBB more or less erect, forming a definite part of the calyx wall. 

 10. Forbesiocrinus; IBB recumbent, almost withdrawn from the exterior, n. Ichthyo- 

 crinus; IBB horizontal, reduced to a small plug, entirely covered by the column. 



Examination of the genera belonging to group A shows that they fall into 

 three subdivisions based upon corresponding conditions of the infrabasals: 



Aa. If we take the calyx of Lecanocrinus as shown by the cross-section 

 (text-fig. 9) it will be seen that it represents the primitive stage of crinoid 



