of the Crinoidea bracliiata. 397 



as P. fusiformts of the Eifcl : here; contrary to what occurs in 

 Pentacnnus, the sutures of the circle in immediate contact 

 with the stem he over the angles, and the middles of the seg- 

 ments over the sides of the pentagon of the stem. We may 

 consequently distinguish the quinqueradiate stems according 

 to their different relation to the division of the base, as iso- 

 meral and anfimeral stems. Stems with a five-lobed nutri- 

 tive canal, whether the base is divided monocyclically or di- 

 cyclically, have always an antimeral position, which appears 

 from the mode of entrance of the five lobes or rays into the 

 calycine cavity. The segments of the basal circle in apposi- 

 tion to the stem are always notched in the middle by one of 

 the five rays of the nutritive canal. 



These various peculiarities in the structure of the stem will 

 have to be taken into consideration in the discussion of the 

 circumstance that quadriradiate and triradiate stems also 

 occur in which the deviation from the quinqueradiate form is 

 recognizable sometimes by their quadrangular or triangular 

 prismatic outline, sometimes by an apparently regularly 

 quadri- or ti-ipartite nutritive canal. 



The change of the regular quinque-partition into a sym- 

 metrical quadri- or tri-partition does not start from the radii, 

 but is peculiar to the dorsal pole of the Crinoid ; it occurs, 

 therefore, both in the monocyclic base and in the lower circle 

 of the dicyclic base. The regular quinquepartite pentagon 

 becomes converted into a symmetrical quadri- or tripartite 

 figure as follows : — Either one of the five directions of divi- 

 sion turned towards the sides of the pentagon, or two directions 

 not belonging to adjacent sides, pass, as it were, out of action. 

 Wliere the division is wanting, a larger plate, of pentagonal 

 outline, is produced, occupying the space of two of the qua- 

 drangular plates of the regularly divided pentagon. The 

 pentagon broken up, in accordance with this law, into four or 

 three parts is said to be symmetrically divided, because it can 

 be separated into two similar halves only in the direction of 

 one of the five axes drawn from the angles to the opposite 

 sides. In the tripartite pentagon this axis cuts the single 

 small plate, and in the quadripartite the middle one of the three 

 smaller and the large plate. 



In the quadripartite pentagon the tendency to equalize the 

 differences of the four inner angles predominates, so that the 

 four sutures meet at the nutritive canal, crossing each other at 

 right angles. This equalization is to be observed in all gra- 

 dations in species oi Melocrtnus. The symmetrical divisibility 

 is not altered by this, as the originally larger segment retains 



