236 



pal;eontolooy of new-yoiik. 



defined species, unless the Agelacrinites of Vamjxem be found to belong to this group, 

 but it appears rather to be a free echinodemi*. 



Such are in brii>f some of the principal characteristics of this family, as at present defined. 

 Several of the lower silurian genera are described as being destiute of arms, and the F;ime is 

 true of a less number of the upper silurian genera. The appearance of arms has been re- 

 garded as a stage of progress in the development of the type ; and, that in this, the Cystideans 

 make a gradual approach to the true Crinoidese, the family finally disappearing before the com- 

 mencement of the Carboniferous period, where true Crinoids are most fully developed. If we 

 admit this explanation, there seems to be no very natural relation between Cystideai and Echi- 

 nidcB proper ; since, if the farther development of the former assumes the crinoid type, we 

 should go behind the Cystideffi for Echinida?, which in truth do not appear till the Crinoideae 

 have passed their fullest development. We know also that crinoids with pentapetalous bases 

 existed in considerable numbers, at least coeval with the earliest cystidean ; and we have at 

 about the same time the asteriad type, so that it is not easy to reconcile these ditferent news, 

 if we have any regard to the order of time in the appearance of types. 



In a treatise, like the present, intended as a record of species and genera in their order in 

 time, there is not space to introduce much disquisition in relation to the history and affinities 

 of the different families of which species are described, more particularly since there is no 

 opportunity of bringing all the species of any family together. 



Prof. Forbes, in his investigation of the British Cystidea;, has presented a very interestmg 

 essay upon the position of this family in the animal series, in which he has shown so many 

 remarkable analogies from his extended comparisons, and from which he has drawn such purely 

 philosophical deductions, that we might rest satisfied with the conclusion. In these deductions 

 he has given the credit due to geology for having brought to light the means of supplying the 

 links in this chain. If, however, we go to geology, which we shall find always necessary, then 

 w'e should take into consideration the occurrence of these, or other connecting links, in the order 

 of time ; and we have no right to neglect that part of the proposition, by taking such fossil 

 forms as may suit our views, without reference to the vast periods of time that have elapsed. 

 The following diagram is given by Prof..FoEBES, as showing the affinities of the Cystideee with 

 the higher Echinodermsf . 



* Prof. Forbes has cited the Agel.icrinites of Vanuxem, but applies the name to a very different fossil, and 

 places it in Lower Silurian, regarding it as congeneric with a species found in Lower Canada by Dr. Bigsby. It seems 

 quite doubtful if the characters of Vanuxem's fossil have been fully understood; and if so, we have the very re- 

 markable fact of a genus of Cystideans common to the Lower Silurian and Devonian systems, which, in these 

 singular and comparatively rare fossils, we can scarcely expect to be true. 



t This diagram is copied as it stands in the memoirs of the Geol. Survey of Great Britain, Vol. ii, part ii, with the 

 exception of the geological position of genera, which has been added. 



