MONOGRAPH OF THE EXISTING CRINOIDS. 585 



In the species which as adults have the division series broad and stout, as for 

 example Crotalometra porrecta, the articular facet on the distal border of the 

 radials increases very rapidly in width, correlatively with the increase in width of 

 the IBri and IBfj, and occupies almost or quite the entire width of the radial at a 

 relatively early age. In the species in which the division series are narrow and 

 widely separated in the adults the widening of the articular facet takes place 

 very slowly. 



Radianal. — The course of development of the radianal, always present in the 

 pentacrinoid young of the comatulids, is the same in all species, but it varies very 

 greatly in the time of its first appearance, and hence in the proportion of the course 

 as a whole which it covers in the different types. 



The radianal is the first plate of the radial series to appear in Comactinia 

 meridionalis and in Promachocrinus kerguelensis. 



It appears simultaneously with, or possibly slightly before, the right posterior 

 radial in Hathrometra prolixa. 



It appears when the radials are almost in contact in Hathrometra sarsii. 

 It does not appear until the period of the development of the IBrj in Antedon 

 bifida, Antedon mediten^anea, and Antedon adriatica. 



It has been observed, but its history is not Icnown, in Comissia littoralis, 

 fLamprometra protectus, Antedon petaszis, Ueliometra glacialis, Anthometra 

 adriani, and Hathrometra tenella. 



The only pentacrinoids of the following which have been studied were too 

 young to show it : C ompsometra loveni, Leptometra celtica. 



In the description of the pentacrinoids of the following species the radianal 

 was not mentioned : Ptilometra miilleri, Crotalometra porrecta, and Glyptometra 

 tuberosa. 



At first the radianal lies in the angle where the posterior and right posterior 

 orals and basals join, just to the left of the right posterior radial plane and mostly 

 below the plane of division between the orals and the basals, being accommodated 

 by a slight cutting away of the right distal angle of the posterior basal. 



The right posterior radial, appearing almost immediately after the radianal 

 and just to the right of it in the same transverse plane, grows much more rapidly, 

 and the result of its increase in size is to shove the radianal along the distal edge 

 of the posterior basal to the left, though the presence of the radianal prevents the 

 development of the right posterior radial toward the left, this side being cut away 

 in a broad curve about the right half of the radianal. 



As the radials increase in size, as rhombic plates, the lateral distal angles of the 

 basals become more and more broadly cut away until finally the distal borders of 

 the basals, instead of being straight and lying in a plane at right angles to the 

 dorsoventral axis, are resolved into two straight lines running obliquel}' upward 

 and inward and meeting in the center at an obtuse angle. 



With the further growth of the right posterior radial the radianal moves along 

 the right distal edge of the posterior basal, outward (to the left) and at the same 

 time upward, its center remaining on the same transverse plane as the center of 



