2/0 SMITHSONIAN INSTITUTION 



species where they are the largest we only see them as pentagons ; in others where they are 

 hidden more deeply under the column the vertical sutures disappear, and only very small 

 triangular points remain ; and in still others the basals cannot be seen at all, the radials rest- 

 ing directly on the column. 



The radianal, resting in the angle formed by two infrabasals and in contact with a radial 

 at each side and above, is necessarily pentangular, but this is not always fully disclosed in a 

 lateral view. It is disproportionately small as compared with the succeeding plate, or with 

 the corresponding radial in other rays ; it has frequently the appearance of an ill-formed 

 plate, either imperfectly developed or in process of elimination ; it often lies below the line 

 of the radials in the other rays, and is entirely covered by the column. See various forms of 

 this plate on Plate XXXV. 



The number of primibrachs seems constant except for an extra one in a single Swedish 

 specimen. The secundibrachs are generally four, with variations of from three to five in 

 occasional rays, but in one species the number seems constant at three. Beyond this the 

 number of brachials in the successive divisions is inconstant ; the branching loses its perfectly 

 isotomous character and becomes more or less unequal. The number of tertibrachs is greater 

 in the outer rami, and varies between 7 and 12, rarely exceeding the latter, and there are 

 other bifurcations beyond them. 



We know nothing of the tegmen from actual observation, but there is no reason to doubt 

 that it is essentially similar to that of the other genera of this group in which it has been 

 found — that is, a flexible disk with the ambulacra running along the surface from the open 

 mouth to the arms. 



The calyx wall had a certain amount of pliability, combined with strength. In the 

 upper part the plates are relatively deep, or thick dorso-ventrally, and fit together like stones 

 in an arch. This arch-like construction of the calyx, with the arms closely folded over it, 

 must have given to the crown great strength in the upper part to resist compression, since 

 notwithstanding the pliant test many of the specimens as found are fairly rotund. 



The column has the general structure which prevails in the columns of the majority of 

 genera of this group, and differs scarcely any among the species in which it is known. It is 

 composed throughout the greater part of rather long ossicles alternating with short ones 

 until near the junction with the calyx, where it gradually widens to nearly or quite the diame- 

 ter of the base, covering the infrabasals and in some species the basals, radianal and part of 

 the radials. In the expanded portion next to the calyx, and sometimes for a little distance 

 below it, the columnals are very thin, and equal. Maximum length of column as shown by 

 two complete specimens obtained after the plates were printed 23 cm. 



Ichthyocrimis is one of the most widely distributed genera of the Crinoidea, being 

 found in various American states from Tennessee to New York, and in England, Sweden 

 and Bohemia; but its geological range is not great, being limited chiefly to the Silurian but 

 with two species in the Lower Devonian. Its direct successor was M etichthyocrinus in the 

 Carboniferous, evolved by the elimination of the radianal through intermediate stages which 

 have not yet been discovered. 



In a genus like this, where the interbrachial system is wholly undeveloped and the 

 brachial arrangement so simple as to admit of but little variation, characters for the satis- 

 factory definition of species are not readily discerned. In all the original specific descrip- 

 tions scarcely anything was given beyond the characters pertaining to the whole group, or to 

 the genus ; so we get very little assistance from any of them. The difficulty is heightened by 

 the paucity of the specimens and their too frequently imperfect condition. Ichthyocrimis 

 has always been one of the rarest forms of fossil crinoids, and collectors have considered 

 themselves fortunate to have any examples of it, however imperfect. The figures given 

 herein represent substantially all the important specimens that have been found during the 



