PROCEEDINGS OF THE ACADEMY OF [1885. 



this species underbasals are known to be absent, and the}' have 

 four in place of five basals. Any difference in the number of 

 basals has been generally considered a good generic distinction, 

 and this make- Xenocrinus, undoubtedly, a good genus. But in 

 which group shall we place it? Together with Ileteocrinus or 

 Glyptocrinus? Together with species in which the interradials 

 rest upon the basals, and which have well developed underbasals, 

 or with species destitute of underbasals, and with their interra- 

 dials. as in Glyptocrinus, resting upon the first radials? 



In Part II we maintained that, as a rule, the presence or ab- 

 sence of underbasals should be considered of more than generic 

 importance, and this we made the principal distinction between 

 Actinocrinidae and Rhodocrinidae. We pointed out, however, 

 that in these families there are several genera, among the earlier 

 types, which are closely connected by transition forms, and shade 

 almost imperceptibly from one into another. We even thought 

 it possible that species of the same genus might possess under- 

 basals in a rudimentary way, while those plates might be totally 

 absent in others. This is not confirmed by our later investiga- 

 tions, but it is nevertheless by no mean- an easy task to separate 

 some of the earlier genera upon this character, as there are fre- 

 quently other important features by which they are much more 

 closeby connected with other groups. In proof of this we need 

 only refer to Glyptocrinus Richardsoni, provided this really 

 possesses no underbasals, a- Miller asserts, and to Ileteocrinus 

 O'Nealli, in which 1 hey are very conspicuous. As the two species 

 are almost identical in every other respect, it would seem doubt- 

 ful policy to refer them to distinct families upon this character 

 alone. S. A. Miller evidently experienced the same difficulty, for 

 his Glyptocrinidae include genera of both forms. Zittel, De Lo- 

 rial, and all preceding writer-, make the presence of underbasals 

 a full family distinction, and all their Glyptocrinidae and Rhodo- 

 crinidae are -aid to have underbasals. 



Diversities in the distribution of the interradial plates of the 

 calyx have been generally taken to be of minor morphological 

 importance; hut at the same time they have been considered good 

 characters for distinguishing genera. S. A. Miller alone has 

 placed in the same genus species, which in this respect show the 

 greatest possible contrast. It is, however, rather singular that 

 he applies this rule only to the - Glyptocrinidae," while in other 



