MORPHOLOGY 2J 



exceeding 5 mm. in diameter of the calyx, while others with a more elongate 

 habitus like Forbesiocrinus attain a width of 8 cm. 



As to surface characters, there is but little variety within the group. With 

 the exception of low radiating ridges on the calyx plates of Pycnosaccus and 

 one species of Lecanocrinns, or following the median line of the ray in some 

 Ichthyocrinus, and of some ornamentation on the arms of a few species, the 

 surface is everywhere either ( 1 ) finely pustulose, composed of distinct pustules 

 or small nodes formed by actual elevations, sometimes coalescing into more or 

 less distinct lines or wrinkles; or (2) smooth or finely granular, in the latter 

 case consisting of granulations formed by the network of stereom, which should 

 be called smooth. 



A. THE PRIMARY PLATES 



I. Abactinal System 

 a. The Column 



The column of the Flexibilia is always circular. Pentagonal, elliptic, or 

 square stems, such as occur in the other orders, are unknown in this. The most 

 marked characteristic is a series of very short and broad columnals, occurring 

 at the proximal end, and diminishing in diameter while slightly increasing in 

 length for a short distance, until the stem becomes cylindrical. There is thus 

 produced a conical enlargement of the stem next to the calyx which is a con- 

 spicuous feature in many of the genera ; it usually changes over gradually into 

 the cylindrical portion, but in one case the enlargement contracts abruptly. 

 This series of almost uniformly short discoid plates represents a diminishing 

 series of successive proximales, without any intercalation of other ossicles. 

 Rarely this process continues throughout the entire column (PI. XIII, fig. 

 no) ; sometimes without any taper (PI. XV, fig. 5). Frequently the columnals 

 increase in length distally until there is a complete reversal in proportions; in 

 other cases the stem maintains an almost uniform diameter and length of 

 columnals until it ends in branching roots formed by radicular cirri 

 (PI. LXXI), with nodal ossicles irregularly distributed. The proximal enlarge- 

 ment is most prevalent in the Carboniferous genera, although found in the 

 Silurian, and in the very earliest species of the group in the Lower Ordovician. 

 In the majority of Silurian genera it is wanting, the stem being cylindrical 

 throughout and largely composed of alternating long and short ossicles, which 

 often alternate in diameter also; the contrast in relative length in some cases is 

 very great (PI. XI, fig. 15a), extending to practical elimination of the short 

 ossicles (PI. VIII, fig. 50) ; in some there is no alternation (PL XV, fig. 5) ; 

 and in one species of Mespilocrinus the columnals become greatly elongated 

 and barrel- or spindle-shaped (PI. V, fig. 5). 



There is much diversity in length; in some genera, such as Calpiocrinus, 

 the stem is very short and thick, being with its expanded root not longer than 



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