262 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
and was supposed to have but one ring of plates beneath the radials. 
Another species, clearly of the same genus, was afterwards referred by Hall 
to [thedocrinus. In the construction of the calyx Lyriocrinus approaches 
Ripidoerinus, which, however, differs very essentially in the arm structure. 
S. A. Miller’s Lyriecrinus sculptus, which in the Revision, Part II., we placed 
provisionally under Archwocrinus, belongs to Diabolocrinus, and is identical 
with D. vesperalis White. 
In this genus we have a good illustration of the exceptional variation 
irom the characteristics of a group, which may be met with among individ- 
uals belonging to it. In JZ. daetylus the first interradials often touch the 
basals only at the anal side, while in L. melissa, they rest upon the basals at 
all five sides. Again, in the former species there is generally an anal plate 
between the interradials of the second row, while in LZ. melissa that plate is 
scarcely ever represented. Departures of this kind within a genus in respect 
to characters which have always been regarded as of the utmost significance 
for distinguishing families and genera show how idle it is to expect absolute 
accuracy in the separation even of very important groups. 
Lyriocrinus dactylus Hatt. 
Plate XI. Figs. da, b, e. 
1843. Marsupiocrinus dactylus —Hat.; Geol. Rep. 4th Distr. N. Y., p. 118. 
1852. Lyriocrinus dactylus — Hau; Paleont. N. Y., Vol. II., p. 197, Plate 44, Figs. la-g. 
1881. Lyriocrinus dactylus — W.and Sr.; Revision Paleoer., Part IL., p. 205. 
Calyx semi-globose; basal regions flattened, the centre abruptly de- 
pressed for the reception of the column. Surface of plates smooth or finely 
corrugated. Infrabasals very small, restricted to the bottom of the colum- 
nar concavity. Basals large, four of them angular above, that of the anal 
side truncate. Radials wider than high, larger than the costals, slightly 
touching each other laterally except on the anal side. Arms ten, simple ; 
heavy at their bases, tapering upwards to one half their former size. They 
are composed from the calyx up of two series of alternate plates with parallel 
transverse faces, and have a shallow groove along the median line ; their two 
proximal plates incorporated into the calyx. Interbrachials: 1, 2,1; those 
of the first and second rows large ; the lower one almost touching the basals ; 
the upper one smaller and somewhat inflected at the upper end, resting 
between the lower arm plates. Another small plate is placed between the 
distichals so that the arms are separated at one side by an interbrachial, and 
