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 Bhodoerimis. In the construction of the calyx Lyriocrinus approaches 

 Ripidocrmus, which, however, differs very essentially in the arm structure. 

 S. A. Miller's Lyriocrimis scidptus, which in the Revision, Part II., we placed 

 provisionally under Archceocrinus, belongs to Diaholocrimis, and is identical 

 with D. vesper alis White. 



In this genus we have a good illustration of the exceptional variation 

 from the characteristics of a group, which may be met with among individ- 

 uals belonging to it. In L. dadylus 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 L. 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 Hall. 

 Plate XI. Figs. 6a, h, c. 



1843. Marsnpiocrinus dactylus — Hall; Geol. Rep. 4th Distr. N. Y., p. 113, 



1853. Lyriocrinus dactylus — Hall; Palseont. N. Y., Vol. II., p. 197, Plate 44, Eigs. \a-g. 



1881. Lyriocrinus dactylus — W. and Sp. ; Revision Palseocr., 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. Eadials 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 



