Mr. F. A. Bather on British Fossil Crinoids. 375 



specimen. Similar minute division of the arms is common in 

 Homocrinus and may equally well have occurred in Pariso- 

 crinus ; and it is with these genera rather than with Cyatho- 

 crinns that Poteriocrinus is in other respects allied. Pinnulate 

 arms of this type may continue to increase in complexity by 

 dichotomy, and then by giving off lateral branches just in 

 the same way as simple arms ; but such complexity does not 

 appear till the Mesozoic Epoch. 



The types of arm hitherto considered are uniserial, i. e. with 

 the joint surfaces of the ossicles more or less parallel to one 

 another. But in pinnulate arms, since every ossicle of the 

 main arm is really an axillare with its two upper joint surfaces 

 unequal in size, there arises a zigzag arrangement of the joint 

 surfaces. This may be so intensified as to produce a biserial 

 arrangement of arm-ossicles ; thus in a given length of arm 

 the number of pinnules is doubled, greatly to the advantage 

 of the animal. This biserial arrangement is chiefly developed 

 in the genera with the two main arms in each ray ; the 

 same physiological end is attained in a different manner by 

 the genera with dichotomous arms. 



The foregoing statement of facts will probably be accepted 

 by all ; from it the following consequences arise : — Neither 

 the branching of the arms by itself, nor the development of 

 pinnules by itself can be taken as characters indicative of 

 divergence, for similar evolution may have taken place along 

 many different lines. As regards arms, for instance, no one 

 would associate Ohiocrinus with Vasocrinus, Holocrinus with 

 Scytalecrinus, locrinus with Homocrinus or Cyathocrinus, or 

 Euspirocrinus with Oncocrinus because these genera happen 

 respectively to have very similar arm-structure. The unim- 

 portance of pinnules on the other hand is best exemplified 

 by the genus Botryocrinus, for while the Swedish species 

 have armlets and not pinnules, the common Dudley species 

 has undoubted pinnules and its arm-arrangement in no way 

 differs from that of Decadocrinus. In recent Crinoids pin- 

 nules differ from arms only by containing the fertile portions 

 of the genital glands ; but it is pretty obvious that in these 

 older Crinoids without pinnules, the genital products must 

 have been borne at the tips of all the arm-branches : hence 

 the physiological difference need not even have been so great 

 as the morphological. There is, however, one character, or 

 rather combination of characters, which seems to be of rather 

 greater importance, and that is the persistence of a simply 

 bifurcate arm bearing first armlets and then pinnules, as 

 opposed to the development of pinnules on a dichotomous arm. 

 Between these two types no connecting-links are evident. 



