408 Prof. Bejrich on the Base {Pelvis) 



Museum the position of the stem-rays eoukl be determined 

 only in two specimens, a Gastrocoma and a Geramocrinus ; in 

 both the position was the normal one of Cupressocrtnus. 

 Myrtillocrinus is also nearly allied to the above-mentioned 

 genera, especially according to Hall's representation of M. 

 americanus. A more accurate examination of this genus 

 w^ould be especially desirable, as it is the only one to which 

 both a quadriradiate stem and a divided inferior basal circle 

 is ascribed. 



In the preceding only the various symmetrical modes of 

 division of pentagonal bases have been taken into considera- 

 tion. Besides these, however, monocyclic hexagonal bases 

 occur, which are divided in Actinocrinus and Hexacrinus into 

 three, and in Dichocrinus into two equal parts. As the hexa- 

 gon is only produced by two radii being pushed asunder down 

 to the base by an interradial space, the hexagon acquires for 

 the Crinoid the signification of a symmetrical hexagon, one 

 side of which as interradial is opposed to the five other sides 

 as radial. vSuch a hexagon is consequently divisible into 

 equivalent halves only in accordance with one axis, which 

 nins across from the middle of the single inten-adial to the 

 opposite radial side, i. e. in accordance with an axis which 

 coincides with the radial axis of the calyx. Consequently 

 the tripartition of the only apparently regular hexagon has to 

 be regarded as a symmetrical one, produced in accordance 

 with the rule that one of the three dividing sutures is placed 

 inteiTadially, and the other two radially. The median suture 

 in the base of Dichocrinus is to be conceived as having one 

 half interradial and the other half radial. 



The observations made on the Crinoidea brachiata led to a 

 comparative investigation of the Blastoidea and Cystidea. 



The Blastoidea all possess a monocyclic, pentagonal, sym- 

 meti'ically tripartite base, and have, above one of the five inter- 

 radial plates, a single opening, which probably represents 

 the vertical aperture of the Brachiata. We may therefore 

 in them also regard the calyx as divided 

 in accordance with a radial axis, and express 

 the division of the base in the same way 

 as in the Brachiata. When viewed in the 

 same relative position that was given the 

 calyces of the Brachiata, the dorsal axis 

 acquires the direction from the left adjacent 

 radius to the right abjacent interradius. 

 This is a position which has not been observed 

 in Brachiate Crinoids with a tripartite base, ^d 



but which has proved to be constant in a great PenUttremites. 



