268 PROCEEDINGS OF THE ACADEMY OF [1881. 



hexagonal. An exception is here found in Dolatocrinus and 

 Stereocrinus^ in which the second radials are quadrangular or even 

 absent, and which should perhaps be placed more properly in a 

 group by themselves. 



The form of the second radials is no doubt of some importance 

 in the structure of these crinoids. In species in which they are 

 quadrangular and linear, the second and third radials together 

 very often fail to attain the size of the first radial plate. In 

 various cases of Dolatocrinus^ Alloprosallocrinus and Batocrinus, 

 the second radials are so small that they are discovered with diffi- 

 cult^' , and are not un frequently in single rays entirely absent. In 

 species in which the plates of the calyx are tuberculous, they are 

 often the only plates which bear no tubercle. All this hints at 

 the conclusion that the second and third radials, which combined 

 take the form of a single bifurcating plate, here take the place of 

 a single plate joined by syzygy, with the epizygial part bearing an 

 arm instead of a pinnule, and that in species which as a rule have 

 only two primary radials, but otherwise agree with some other 

 genus, the joints became perfectly anchylosed. Such was evi- 

 dently the case with Dolatoci'inus and Stereocrinus, Eucrinus 

 and Anthevwcrinus, Lecanocrivus and Pycnosaccus. In Platy- 

 crinus, which also has only two primary radials, the division 

 appears yet frequently in form of a shallow groove at the surface 

 of the plate, where the earlier Platycrinidse have a regular 

 suture. 



In species with more than ten arms, the rays are generally com- 

 posed of two main divisions, of which each side gives off arms in 

 opposite directions. The onl}' exception is Steganocrinus sculjytus 

 Hall, in which the ray is undivided (PI. 18, fig. 3). In this 

 species, the third primary radials, like all succeeding plates 

 (radials of superior orders), take the form of pinnule-bearing 

 plates, which, instead of bifurcating, give off" laterally arms in 

 the same manner as the others do pinnules. 



This is of interest, as it leads to the conclusion that probably 

 the secondary radials — the distichalia of Miiller — made their 

 appearance in the young crinoid in form of a pinnule given off 

 from the radials which at first formed the only arms of the ray. 



We have already shown in our general remarks on the family 

 that the higher orders of radials were in the young animal free 

 arm plates, and we have proved by many examples that the arms 



