398 Prof. Beyricli on the Base {Pelvis) 



its pentagonal form ; but the sutures are displaced, and no 

 longer strike the middle of the sides. A similar equalization 

 of the inner angles might also be possible in the tripartite 

 pentagon by the displacement of the two sutui-es bounding 

 the unpaired segment; but this appears to occm' but rarely. 



In the preceding three figures, a shows the symmetrically 

 tripartite and b the symmetrically quadripartite pentagon ; 

 c the latter with the inner angles equalized. The dotted lines 

 in c are in the position of the imcUsplaced sutm'es. 



Simple as these conditions are, they were not, when first 

 observed, either correctly interpreted or particularly valued. 

 We may see this from the erroneously indicated divisions, 

 such as are represented in the figures in Goldfuss, Petr. Germ. 

 Taf. 58. fig. 3, or in Johannes Miiller, I c. Taf. 6. fig. 1 a. 

 That both the tripartite and the quadripartite pentagon are 

 only modifications of the quinquepartite, and formed in ac- 

 cordance with a definite law, was first explained by L. von Buch 

 in his memoir on the Cystidea ; at first, also, he had a notion 

 that there might be a certain connexion between the occmTcnce 

 of a symmetrically divided base and a lateral position of the 

 vertical aperture ; but by the further caiTying out of this idea, 

 he arrived at false conclusions. His opinion was that the 

 axis in accordance with which the base is divisible into two 

 similar halves, if prolonged meridionally round the Crinoid, 

 must strike the excentrically placed vertical apertm'e ; and he 

 went so far as to believe that a central vertical aperture can 

 occur only where the base is of regular quinquepartite struc- 

 ture (Ueber Cystideen, p. 5). It would almost appear that at 

 the time when he was endeavouring to decyi^her the natm'C of 

 the Cystidea, this observer, otherwise so acute, had never seen 

 the well-preserved calyx of a Brachiate Crinoid with a penta- 

 gonal tripartite base. He depends chiefly upon the genus 

 Actinocrinus (Cystideen, Taf. 2. fig. 9), which, however, does 

 not possess the pentagonal base ascribed to it, but an hexa- 

 gonal one ; and for Platycniius he refers to the figures of 

 Johannes Miiller, in the memoir on Pentacrinus (Taf. 6), in 

 which there is nothing to be seen but an erroneously figured 



