374 Mr. F. A. Bather on British Fossil Grinoids. 



which that group must have passed. What appears to have 

 been the case is that this dichotomy once started was rapidly- 

 continued, so that the next stage presents us with arms 

 dividing equally at fairly regular intervals : Bendrocrinus, 

 Homocrinus, Iocrinus, Merocrinus, Cyathocrinus, and a few 

 genera closely allied to these have arms of this type (PL XV. 

 figs. 2, 3, 4) . As every additional bifurcation is an advantage 

 to the animal, it is reasonable to suppose that an arm with a 

 large number of terminal branches is more advanced in deve- 

 lopment than one with few ; some species of Cyathocrinus for 

 instance have a far larger number of bifurcations than others. 



The next stage shows one limb of each bifurcation after 

 the first becoming smaller than the other ; thus, instead of 

 one regularly dichotomous arm, there arise two main arms 

 with bifurcating branches given off alternately on either side. 

 At first these branches attain quite or almost the length of 

 the main arm, e. g. Heterocrinus , Ohiocrinus, and Belem- 

 nocrinus (PL XV. figs. 5, 6, 7, 20). The advantage of 

 such an arrangement over simple dichotomy is not at 

 once obvious, but it seems to be that the food-grooves are 

 thus more evenly distributed over the area covered by the 

 extended arms ; further we may suppose the arms to be more 

 easily wielded when there is a stout median ridge. At any 

 rate further evolution takes place in this direction ; the main 

 arm becomes fringed with armlets which again bear small 

 lateral branches, e. g. Vasocrinus, Barycrinus, and some 

 Botryocrini (PL XV. figs. 9, 10). When finally the arm- 

 lets become small, cease to branch, and are regularly placed 

 on alternate sides of successive joints, they are called Pinnules ; 

 a species of Botryocrinus from the Wenlock Limestone is the 

 earliest form known to have reached this stage, which is 

 again exemplified in Becadocrinus, Scytalecrinus, Graphio- 

 crinus, and other allied genera (PL XV. figs. 13, 14, 15). 



This, however, is not the only way in which pinnulate arms 

 have arisen ; pinnules of course always originate in the same 

 way, but their arrangement on the arm may be different. 

 Anomalocrinus (PL XV. fig. 11) is enough to show that pin- 

 nules may be developed directly on a dichotomous arm, 

 although it is so anomalous a form that it leads no further. 

 But the pinnules of Scaphiocrinus and Poteriocrinus also 

 appear to have arisen on dichotomous arms without inter- 

 fering with the dichotomy (PL XV. fig. 12) ; how this 

 may have happened is seen in a very beautiful specimen of 

 Cyathocrinus in the British Museum in which the arms are 

 very finely divided at the tips and so closely resemble pinnules 

 that even Mr. Springer was deceived when he examined the 



