MONOGRAPH OF THE EXISTING CBINOIDS. 191 



Promachocrinus and Thaumatocrinus . 



The calyx of the pentacrinoid larva of Promachocrinus is very robust, more so 

 than that of any other comatulid, and is characterized by pronouncedly convex 



FIG. 122. LATERAL VIE w OF A YOUNG SPECIMEN OF PENTAMETROCRINUS, SP. FROM ICELAND, SHOWING THE RELATIONSHIPS or 



THE CENTRODORSAL, BASALS, RADIAL9, AND ISTERRADIALS, AND THE PERISOMIC PLATES OF THE DISK. 



sides and great breadth across the radials. This is probably due to a more than 

 usually rapid growth of the internal organs, intensified by a diminution in the rapid- 

 ity of the growth of the calyx plates resulting from the coldness of its habitat. 



