10 GEOLOGICAL MEMOIRS. 



the Pentremites in two marginal longitudinal rows are not, as was 

 formerly supposed, intended for the passage of soft membranous 

 feelers, like the holes on the tentacular spaces of the Echinidse, but 

 they are canals for nourishing appendages composed of portions of 

 calc-spar, and formed in the same manner as the pinnulse on the arms 

 of Crinoids." 



With this the approximation of the Pentremites to the Echinites 

 hitherto generally received falls to the ground, as no longer capable 

 of being maintained*. The Pentremites are genuine Crinoids, which 

 instead of a limited number of compound arms, are provided with 

 numerous, definitely-arranged, simple, undivided appendages, com- 

 posed of pieces of calc-spar. 



They form a peculiar group among the Crinoids, characterized not 

 only by the numerous, simple, arm-like appendages, but also by the 

 five large openings placed round the central vertical opening, and by 

 a peculiar simple disposition of the plates of the cup. 



In the absence of true compound arms, the Blastoidese (the family 

 name already given to them by Say) agree with the Cystidese of L. 

 von Buch. The scars (Narben) which occur near the mouth in some 

 of the latter may have been the points of attachment for similar 

 simple appendages ; but many arms could not have been placed there, 

 as has already been distinctly stated in the celebrated monograph of 

 the family. 



In conclusion, we must say one word in regard to the physiological 

 purpose of the arm-like appendages now described in the Pentremites. 

 The pinnulse of the Comatulae and Pentacrini with which these ap- 

 pendages may be most suitably compared, have, according to Johannes 

 Miiller, a twofold purpose. In the first place, they serve to seize and 

 to convey to the mouth the food necessary to the support of the ani- 

 mal ; and in the second, the male and female reproductive organs are 

 situated at the base of these pinnulse. 



It is in every respect very probable that the appendages of the 

 Pentremites may have served for the first purpose, and the definite 

 direction which they all assume towards the central opening in the 

 vertex, which undoubtedly corresponds to the mouth, confirms this 

 view. On the other hand, it is impossible that the reproductive 

 organs can have been placed at their basis, if, as seems necessary, we 

 consider the five openings on the top surrounding the central one of 

 the mouth as the ovarial openings. 



That these arm-like appendages of the Pentremites should not have 

 been previously observed, does not seem remarkable when their ex- 

 tremely delicate structure is considered, and we must rather ascribe it 

 to some peculiarly favourable conditions during petrifaction that they 

 have been preserved even in a single case. 



Though not immediately connected with what precedes, I must 



* That the formation of the so-called ambulacra! spaces themselves, and the 

 position of the holes upon them, is entirely different from that of the tentacular 

 spaces of the Echinites, can only be indicated in this place. The more complete 

 exposition of the fact must be reserved for a connected work on the genus Pentre- 

 mites, for which I have collected rich materials in North America. 



