UINTACRINUS: ITS STRUCTURE AND RELATIONS. 



71 



while others have only 1, or 3, A, 5, 6, or 7 plates, variation from 2 to 4 or 

 5 occurring in the same individual. Here the increase is always by upward 

 growth. I give a series of figures showing the manner of such increase, as 

 well as the variations in form and arrangement in a number of specimens. 



15 



Variations in intersecuiidibracb spaces. 11 and 12 are from specimen No. 243; 13 from No. 57; 14 

 from No. 96; 15 from No. 29; 16 from No. 24; 17 and 19 from Up; ]8 from No. 57^; 20 from No. 20; 

 21 and 22 from No. 33 a ; 23 from XllI a ; 24 from VII ^. 



As already stated, these two kinds of plates belong to the category of 

 supplementary plates, which serve to fill up spaces and increase the size of 

 the visceral cavity. They were described by Wachsmuth and Springer in 

 the Monograph of the Crinoidea Camerata, p. 110, where we stated, as a con- 

 clasion based upon the study of Palaeozoic forms, that they "increase by 

 multiplication in the growing animal, primarily in an upward direction, but 

 partly by intercalation, — secondary plates being introduced between the 

 primary ones." Accordingly, considerable irregularity is to be expected in 

 their number and arrangement, and we should expect that this irregularity 

 would be more pronounced in the older individuals ; that there would be, in 

 general, an increase in number corresponding to the growth of the indi- 

 vidual, but not any absolute regularity in such increase. This agrees with 

 the observed facts among many Palaeozoic Crinoids, especially among the 

 Flexibilia, and many Camerata. A fine illustration of it is seen in a species 

 of " Forhesiocrinus " * from the lower Carboniferous, of which I have a large 

 series of well-preserved specimens showing the stages of growth from very 

 young to mature, which furnish the following data on this point : 



* I have no doubt that the name '' Forbesiocrinus'^ will have to be given up, as suggested by Bather. 

 The English species taken by de Koninck as type is a Taxocrinus pure and simple, while the two specimens 

 figured by him from Belgium surely belong to the type for which the genus Onychocrinus has been estab- 

 lished. In 1888, after seeing Phillips's type specimen of T. nobilis in the Gilbertson collection, and also good 

 specimens of Sagenocrinus from Dudley, I placed a label " Sagenocrinus '' in our tray of Forhenocrims 

 Agassizi, and I think it will be found that all the American species of Forhesiocrinus will fall readily under 

 that genus, of which I have found a new species from our Upper Silurian also. 



