MORPHOLOGY 55 



figs. ia-d; ^a-d). In some genera which are usually without any such plates a 

 straggling one is occasionally to be seen, and it is quite probable that if we 

 could see the interior of all the specimens, we should find many instances of 

 sporadic plates which have not come to the surface (PI. XXXIX, fig. gb). It 

 is a fact thoroughly established that these plates multiply with the growth of 

 the individual; but it would seem that in those forms in which the rays are 

 more or less contiguous, and tend to arch over the interbrachial areas, the 

 plates are crowded by the growth of brachials, and are to a certain extent re- 

 duced or suppressed at the outer surface of the calyx. In genera like Taxo- 

 crinus, Forbesiocrinns, and Uintacrinus, the variation of the interbrachials 

 with age is very marked, young individuals in some species having none at all, 

 while the adults are profusely supplied (Pis. XXVIII, LV, and Mem. Mus. 

 Comp. Zool. Harvard, vol. 25, no. 1, 1901, pp. 73-85). In Wachsmuthicrinus, 

 which has no anal plate, the interbrachials are still more variable, being present 

 or absent in the adult of the same species. In Nipterocrinus, Pycnosaccus, and 

 Cholocrinus the perisome extends down to the radials as a plated integument, 

 without developing any definite interbrachial plates. 



Notwithstanding the irregularities in some cases as above mentioned, the 

 varying development of the interbrachial plates affords important characters 

 for classification in this group. In some genera the perisome did not pass down 

 between the rays in such a manner as to form any permanent plates in the inter- 

 brachial areas; in others they were abundantly developed. Both forms extend 

 from the Silurian to the Carboniferous; the first being characteristic of a little 

 group of genera which may be taken as the typical Ichthyocrinidae — Ichthy- 

 ocrinus, Synaptocrinus, Metichthyocrinus, etc. The tendency in the paleon- 

 tological history of the group is the same as in the individual, viz., toward an 

 increase of interbrachials. But few forms without such plates are found after 

 the Silurian, while in the Carboniferous their presence in considerable numbers 

 is the general rule. On the other hand, while the latter stage was fully at- 

 tained in the Silurian genus Sagenocrinus, it was the exception in that age. 



There is no family in this group in which the rays are completely 

 separated by interradials as in the Rhodocrinidae and Reteocrinidae of the 

 Camerata. But in some of the later species of Taxocrinus (Pis. LIX, LX) 

 a tendency is developed for the interradials to pass down and connect by short 

 faces with the basals. Somewhat analogous to this is the isolated occurrence 

 among the Inadunata of corresponding age of a species in which the radials 

 project downward between the basals until they meet the infrabasals (See 

 Wright, Trans. Geol. Soc. Edinburg, X, pt. 1, 1912, pi. 6, figs. 4, 7; pt. 2, 1914, 

 pi. 18, fig. 8). 



