288 Messrs. P. H. Carpenter and R. Etheridge, Jun., 



and more fully developed forms are at a very low stage of 

 organization* as compared with most other Crinoids ; so that 

 it is natural to suppose they would present a considerable 

 amount of variability, both local and general. 



Although it may be thought that the variations we have 

 described exceed the widest limits allowable on these grounds, 

 we prefer rather to ask for an extension in this direction than 

 to multiply species in a manner which appears both artificial 

 and unnecessary. 



When all the above facts are taken into consideration, it 

 seems to us hardly possible to doubt that the specimens we 

 have described represent various stages in the development of 

 a Palseozoic Crinoid. In the smallest examples there is a 

 relatively large oral pyramid, and the uniformly sized radial 

 plates were not perforated by a central canal. The axial 

 cords lay at the bottom of grooves in their upper surfaces, 

 just as in the young Pentacrinoid larva of Comatula (with 

 closed oral pyramid) and in the mature stages of many Palao- 

 crinoids. Next we find specimens in which there are distinct 

 canals developed for the axial cords, and the articular facets 

 of the radials gradually come to exhibit their characteristic 

 markings. At the same time the sizes of the different radials 

 become more or less unequal, and the orals relatively less 

 prominent, though still resting directly on the radials. 



Lastly, in the best-developed examples the radials have 

 strongly marked articular facets — some of them (never all) 

 being axillary and bearing two arms, while the orals have 

 entirely disappeared as an integral part of the calyx. In fact, 

 one would scarcely expect to find them retaining their em- 

 bryonic condition of a closed pyramid on the top of a calyx, 

 the radials of which had reached such a high state of deve- 

 lopment. 



The very complete fusion of the orals in the specimens 

 represented in figs. 8-10 (PI. XVI.) appears to show that they 

 remained united until a comparatively late stage, and so 

 closed in the tentacular vestibule, in the floor of which was 

 the opening of the mouth. 



* Beyrich has pointed out (Crinoideen des Musclielialks, pp. 43, 44) 

 that in young individuals of Encrinus the sutures between the basals are 

 invisible, though those between the radials are distinct enough. This is 

 the case in nearly all our specimens of Allagecrinus, both young and old. 

 It may also happen in the young Encrinus that one of two arms on 

 the same axillary may remain rudimentary, whUe the other develops 

 first. The inequality in size of the radials in Allagecrinus and of the 

 arms which they bear is even a lower condition than that noticed by 

 Beyrich in the young E7icrinHs. There is no similar stage in the young 

 of recent Crinoids, in which all the radial plates are equal from the first. 



