VIII. Cijathocrinus. 223 



width of 4*2 millim., or a little more than half the probable 

 width of the stem. The facet for the stem is I'adiately 

 striated. The stem was tlierefore probably like that of G. 

 acinoliihus. 



In the general shape of the cup and in its probable stem- 

 characters the species resembles tlie group of C. acinotabus. 

 The shape of the cup is most like that of G. acinotuhus, and 

 indications of the ridge that is here so marked may also occa- 

 sionally be seen in that species. The ornament, however, 

 more resembles that of G. vishycewiisj var. monilifer. In the 

 notched facet and imperforate articulation this species differs 

 from most Silurian Cyathocriax ; but such a stage of deve- 

 lopment is occasionally presented by G. slriolatus^ which 

 belongs to the same group. The large size of the cup is a 

 character of no great importance, but affords a ready means 

 of distinguishing the species in British collections. 



General Remarks on the Genus. 



The British specimens do not throw much light on the 

 morphology of the genus, so that there are very few points to 

 which attention need be here directed. 



Growth of the cup. — From the various measurements of the 

 plates of G. acinotuhus given on p. 217, it seems to follow 

 that the facets of the radials, and consequently the arms, are 

 wider in proportion in the young than in the adult ; also that 

 the radials are proportionally wider below in the young. 

 This latter fact harmonizes with the statements already made 

 in general terms by Messrs. Wachsmuth and Springer * and 

 Mr. S. A. jMillerf as to the infrabasal and basal plates of 

 Crinoids being more largely developed in the young than 

 the other plates of the cup. That statement too, it may be 

 mentioned, is confirmed by the measurements made of the 

 present species. It is extremely interesting to note how 

 closely the growth of this Silurian Crinoid agrees with the 

 growth of the Pentacrinoid larva of a recent Anteclon \. But 

 it would be advisable to tabulate the measurements of large 

 sei'ies of many other species before laying down any general 

 laws as to the growth of Silurian Inadunate Crinoids. 



The Axial Ganal of the Arms. — So much Avas said about 



• Rev. I. 19, Proc. 1879, p. 242. 



t " Structure &c. of American Palaeozoic Crinoids into Families," 

 Amer. Geol. vol. vi. p. 282, line 11, ^l'ov. 1890 ; and ' American Geology 

 and Palaeontology,' p. 212, Cincinnati, 1889. 



X See W. B. Carpenter, " Researches on the Structure, Physiology, 

 and Development of ^«^erfo?2 {Comatula, Lamk.) rosaceus," Phil. Trans. 

 18G6, pp. 727, 29, 731. 



16* 



