VON BUCH ON THE CYSTIDEA. 27 



on the surface of the plates of all the tessellated Crinoidea and de- 

 veloped in almost all the C5^stidea * — that we are enabled to deter- 

 mine accurately its true nature. Miller supposes, and I think with 

 reason, that these strise, folds or ridges were owing to a membrane 

 projecting between the plates and coating their external surface — 

 perhaps also producing the striae of growth. The existence of such 

 a membrane would at once explain the reason why the folds are 

 invariably at right angles to the edge of the plate ; it would also ex- 

 plain why they affect only that triangular area enclosed between the 

 centre of the polygonal plate and two angular points towards the 

 side and are prevented from extending to the adjacent triangles, and 

 why they pass on from one plate to another, producing those singular 

 striated rhombs on the external surface, and producing these only 

 on the exterior without any trace of them existing on the inside. It 

 may also possibly admit of a similar explanation, that when at last 

 these folds seem to pass off into pores such apparent pores are by no 

 means orifices for ambulacra, which is proved by their never pene- 

 trating the plate as those do which proceed towards the angular points, 

 since they may be the extremities of hollow channels that are formed 

 between these striae or folds and the general surface of the plate. 



If, notwithstanding the small claim which it has been shown that 

 the Caryocrinites possess to be ranked among the Crinoidea, they 

 may still seem to approach the Poteriocrinites in their cylindrical 

 form, their tall lateral plates, and the scapular plates alternating 

 with them, yet will a closer investigation soon show that these two 

 cannot be classed together, the former group remaining separate 

 and detached, standing between but without absolutely uniting the 

 families Crinoidea and Cystideo, 



The settled dominion of the nxxmbeYjive, which so singularly ob- 

 tains in all organised nature, may be observed in all the Radiaria, but 

 is especially exhibited in the Crinoidea-, and the separate parts show, 

 by reference to this number, the place to which they belong. How- 

 ever various species of Actinocrimis and Pentacrinus multiply them- 

 selves to an almost unlimited extent, this is merely effected by the 

 continued doubling of five arms which are developed from the in- 

 terior of a cup -shaped body; and if, on the other hand, Platycrinus 

 or Actinocrinus seem to rise from only three basal plates, it is easy 

 to show in what way two of these are combinations formed of a pair 

 of plates, so that here also the basis of the cup must be considered 

 as made up of five plates placed in contact. 



Not so in the case of the Caryocrinites. In them we find no trace, 

 no indication which can in any way lead to a grouping into five parts, 

 for everything, even to the most minute point, shows that the number 

 six is dominant, a number which cannot in any way connect itself 

 with five. The base of the cup consists of four unequal plates, 



* The same appearance is yet more prominently developed in the highly pro- 

 jecting ridges which divide the hody oi Jctmocrinitos ^0-dacfi/lus and A.poli/dac- 

 tijlus (Miller, p. 98-100) into deep compartments, from three to five in nnmhcr. 



