74 F. A. BATHER, CRINOIDEA OF GOTLAND. 



Now, just as it is not far from a complete arm of the Gastocrinus or Euchirocrinus 

 type to ;i simple dichotomous arm such as that of Heterocrinus exilis 1 ), so also it is nol 

 far from an axil-arm of the above type, through such a sta ge as that presented by Ecteno- 

 crinus 2 ), to a regularly pinnulate arm such as that of Decadocrinus*). In one species 

 Innu Gotland (C. pinnulatus) this development has actually taken place; in this speciea, 

 however, certain peculiarities seem t<> show that it is out »it the regular evolutionary series 

 as sketched above, and its relationship will be more fully diseussed låter on (p. 99). 



The pinnulate arm is not, however, the goal to whicli the majority of the Calceor 

 crinidae tend; there is another tendency in obedience to which is evolved an arm unlike 

 that of any other Crinoid. We have already seen how the ossicles on the extreme left 

 are magnified and become raain-axils; much in the same way, the left-hand armlets of 

 each axil-arm always tend to be bigger than those on the right; and gradually the armlet 

 on the extreme left and the Betabrachs that support it become larger, while the rest of 

 the arm becomes correspondingly smaller. To such a pitch is this eventually carried that 

 the armlet on the left, the left-hand Betabrachs, and the Alphabrachs appear to form one 

 continuous and regular arm; while the fact that right-hand Betabrach and Gammabrach 

 series exist can only be ascertained by very minute scrutiny. The stages of this evolution 

 are most clearly marked by the species of Calceocrinus herein described. The important 

 point to notice is that the right-hand Gammabrachs and the series that follow them are 

 morphologically the main part of the axil arm; and yet this part, if noticed at all, has 

 been described as merely a pinnule. 



The diminution of the main arm and right-hand armlets has proceeded to a far 

 greater extent in the more specialised European Silurian species, than it seems to have 

 done in the American species, even from låter periods and belonging in niany cases to 

 the genus Halyniocrinus. It appears in fact that yet another tendency was emphasised in 

 the American seas, and that was the tendency of the axillaries (not the main-axils) t" 

 be somewhat stouter owing to a thick callosity at their upper end». This is characteristic 

 of Calceocrinus Fletcheri, i ) but gradually lessens in other European species as the arms 

 become differentiated. 



The questions suggested by these differences of tendency in conjunction with geogra- 

 phical separation are profoundly interesting, for they are connected with the most im- 

 portant problems of modern biology. Without a more exhaustive study of the American 

 ( Jalceocrinidae than is at present possible for me, I dåre not suggest an answer; but I 

 vvould warn my readers against taking the word »tendency» in any teleological sense. 

 The forces that produce differences of this nature, are, we can hardly doubt, much the 

 same as those that are changing a population gathered from diverse races and niany lands 

 into a solid American nation with easily recognised traits. 



7. Stem. In its proximal region the stem curves towards the crown, and this 

 curvature, as has also been noticed by Ringueberg (op. rit., p. 399) for Euchirocrinus 



') Ann. Mag. Nat. Tlist., ser. G, Vol. V, Plate XV, fig. b a. 



-) Op. cit. Tab. cit., lig-. 7. 



3 ) Op. cit. Tab. cit., fig. 14. 



4 ) Salters Cat. Camb. and Sil. Foss. Cambridge, p. 119, figure. 



