248 PROCEEDINGS OF THE ACADEMY OF [1879. 



alternately arranged. The plates of each side taper toward the 

 end and infold over the furrow, covering it as perfectly, and in 

 the same manner as in the two former cases. Angelin gives no 

 description, but in his table of contents, he calls the successive 

 plates from each side " pinnulse." 



It is easily seen that the covering to which we refer in the above- 

 mentioned genera, is constructed upon exactly the same principle. 

 It makes but little difference whether there are two single plates 

 or two rows of plates alternating with each other, and it seems 

 evident to us that if in one case they represent pinnulse, we may 

 well consider them to be the same in the other. It is here im- 

 portant to note that in those genera in which the ambulacral groove 

 is thus covered, no regular pinnules have ever been observed, and 

 moreover the construction is such that no additional pinnulse could 

 have existed; while on the other hand no covering has ever been 

 discovered in forms with true pinnulse. From our observation 

 we are of opinion that the pinnulse generally in the Palaeocrinoidea 

 served partly as a cover or protection for the furrow. In a speci- 

 men of Graphiocrinus tortuos'ts, Hall, in our collection, the pin- 

 nulse cover the furrow so perfectly that we were for some time led 

 to consider them as a solid integument composed of regular spi- 

 niferous plates. With a good magnifier, however, we clearly de- 

 tected the joints of the pinnulse, which are here so placed that 

 the little spines with which their sides a;-e provided stand up erect. 

 In the Actwocrinidse and Platycrinidse the pinnulse are long, com- 

 paratively slender, and so closely arranged side b} r side that it 

 appears as if they were connected laterally, which we think is 

 realty the case in some groups. When the arms are closed, the 

 two series of pinnulse of one arm are laid upon each other so 

 neatly, that the arm furrow must have been thereby perfectly shut 

 off from the surrounding water. No additional covering has yet 

 been observed in these genera, and it was evidently unnecessaiy. 



All this seems to point to the conclusion that the pinnulse had 

 the same functions, partly at least, as the alternate plates in Gya- 

 thocrinus, etc., and as both have the same position, and evidently 

 could be opened and closed by the animal, we do not hesitate to 

 consider the latter as the homologue of the former, or in fact as 

 rudimentary pinnulse. 



