3{>4 Prof. Bejrich on the Base {Pelvis) 



nient which Bronn adopted for the Crinoidea, in 1860, in his 

 ' Khissen und Ordiuingen des Thierreichs,' divides the Bra- 

 chiata, after Miiller's example, first into the Tessellata and 

 Articulata, and makes smaller sections in the arrangement of 

 the genera within these divisions, exclusively from characters 

 presented by the various composition of the base. The ar- 

 rangement is artiiicial, and by no means brings the true rela- 

 tions of the Crinoidea in all cases to a correct and definitive ex- 

 pression ; but it has great advantages over other, older attempts 

 at dividing the Crinoidea into families. 



In accordance Avith their composition, the bases are to be 

 distinguished as those which possess a regular quinquepartite 

 structure, and those in which the regular quinquepartite com- 

 position becomes converted into a symmetrically quadri- or 

 tripartite one. This conversion is subject to definite rules, the 

 exposition of which is the principal object of this memoir. 

 This must be preceded by a consideration of the regularly 

 quinquepartite base in connexion with the structure of the stem. 



The composition of the regularly quinquepartite base fol- 

 lows the following law : — Beneath the circle of the first radial 

 joints a circle of equally numerous segments is inserted in 

 alternating positions, the sutures of which strike the middle of 

 the radial segments, so that the segnients of the basal circle, 

 when considered in their relation to the calycine radii, occupy 

 an interradial position. A second circle of segments may 

 follow, alternating in the same way with those of the first 

 circle ; so that their position beneath the calycine radii is 

 maintained, and they would form a continuation of these if the 

 first basal circle were deficient. In the pedunculate Crinoids 

 the base is completed by the five sutures of the first basal 

 circle when this alone is present, or of the second additional 

 basal circle reaching down to the nutritive canal which unites 

 the base with the stem. When only a single divided circle is 

 present (as in Ciqjressocrinus) , the five segments of the circle 

 apply themselves around an undivided central segment which 

 is perforated in the middle by the nutritive canal ; such a 

 central segment, as it essentially participates in the formation 

 of the calycine cavity, must be referred to the base, and be 

 regarded as the representative of the second basal circle. 

 Pedunculate Crinoids with two divided circles in which a 

 similai behaviour occurs in the second circle are unknown, as 

 are also Crinoids with three divided basal circles*. It is only 



* Hall's genus Dendrocrinus, to which that author ascribes three basal 

 circles, canuot come into consideration here, as both the figure and descrip- 

 tion show that the statement rests upon the erroneous interpretation of 

 imperfectly preserved remains. 



