VIII. Cyathocrinus. 223 
width of 4*2 millim., or a little more than half the probable 
width of the stem. The facet for the stem is radiately 
striated. ‘The stem was therefore probably like that of C. 
actnotubus. 
In the general shape of the cup and in its probable stem- 
characters the species resembles the group of C. acinotubus. 
The shape of the cup is most like that of C. actnotubus, 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 C. vishycensis, var. monilifer. In the 
notched facet and imperforate articulation this species differs 
from most Silurian Cyathocrini; but such a stage of deve- 
lopment is occasionally presented by C. striolatus, 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.—F rom the various measurements of the 
plates of C. acinotubus 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. 8. A. Miller + 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 Antedon tf. But 
it would be advisable to tabulate the measurements of large 
series of many other species before laying down any general 
laws as to the growth of Silurian Inadunate Crinoids. 
The Axial Canal of the Arms.—So much was said about 
* Rev. I. 19, Proc. 1879, p. 242. 
t+ “Structure &e. of American Paleozoic Crinoids into Families,” 
Amer. Geol. vol. vi. p. 282, line 11, Nov. 1890; and ‘ American Geology 
and Paleontology,’ p. 212, Cincinnati, 1889. 
} See W. B. Carpenter, “ Researches on the Structure, Physiology, 
and Development of Antedon (Comatula, Lamk.) rosaceus,” Phil. Trans, 
1866, pp. 727, 29, 731. 
1G? 
