NO. 1846. ON CERTAIN ELEVTHEROZOIC PELMATOZOA—KIRK. 87 



calyx of from 61-75 mm., 59 per cent are monocyclic. The high 

 percentage of Form D in the young and the low percentage of the 

 same type among the adult specimens is evidence amounting almost 

 to proof that there is a change in the structure of the base in the 

 ontogeny of the animal. 



It may be argued, and this seems to be the attitude assumed by 

 Springer, that if the change from dicyclic to "monocyclic" took 

 place in the ontogency of certain individuals, the same process should 

 be effective in all cases. It is highly unreasonable to look for the 

 transmutation of one type into the other in the case of each and 

 every crinoid. Such a universal acquisition of this character could 

 only obtain where the tendency had become most thoroughly impressed 

 upon the race through its acquirement by a great number of succes- 

 sive generations. In Uintacrinus the tendency toward the fusion of 

 infrabasals with the centrale had its inception at no very remote date. 

 This is clearly shown by the fact that the period of fusion, although 

 variable as to the time of occurrance, in most cases comes rather late 

 in the ontogeny of the animals. 



An examination of the tables in Mr. Springer's monograph on 

 pages 78-85 will show even more clearly than the percentages already 

 given how gradually and at what different stages in development the 

 change from one form to the other took place. These facts prove 

 that the fusion of the infrabasals was a comparatively new thing, 

 and as a result was not uniformly acquired by the different individ- 

 uals. Everywhere we find that certam individuals are more acceler- 

 ated in development than others, A structure that appears in one 

 Uintacrinus when but 25 mm, or less in diameter might well be re- 

 tarded in another until the animal had attained its maximum growth. 

 Another factor that may be considered is that of sexual difference. It 

 is a matter of common knowledge that one sex is frequently more 

 accelerated in development than the other, and I have been assured 

 by Prof. R. T. Jackson that such is the case among living Echino- 

 derms, at least in some forms. This differential sexual acceleration 

 I would not lay much stress upon for the reason that probably the 

 females so greatly exceeded the males in number that any such differ- 

 ence would but slightly affect the general result. This probable pre- 

 ponderance of the females over the males will be discussed later. 



At any rate we can readily conceive that such a fusion of the cen- 

 trale with the infrabasals, taking place first in the adult crinoids, 

 gradually was pushed back farther and farther in the ontogeny of the 

 animals, and, finally, may have taken place immediately upon the 

 formation of the centrale, following the detachment of the crinoids 

 from their stems. This acceleration in development, resulting in the 

 acquirement of a "monocyclic" base at various stages of growth 

 answers perfectly the objection raised by Springer on page 26 in 



