324 SMITHSONIAN INSTITUTION 



whereas in this the plates are well-rounded, their curvature breaking the otherwise circular 

 outline of the cross-section, and the sutures are marked by a strong marginal elevation giving 

 a sharply imbricated appearance. The arms are heavy, abut very closely, and taper but little 

 to the point where they disappear by infolding. The last two features are shared by M. tiarae- 

 formis, which also has 4 secundibrachs as a rule. The interlocking of the rays is very perfect 

 at least as far as the upper secundibrach, and there is to be seen at about this level, in at least 

 six interbrachii of the principal specimen, a very small intercalated plate, which probably indi- 

 cates the vertical limit of the calyx (PI. XXXIX, figs, ga, b). 



M. clarkensis was described from a much flattened and imperfect specimen, and from 

 the description and figure 1 supposed it to be a synonym of M. tiaraeformis of the same 

 horizon. Fortunately I came into possession of some excellent material from the same vicin- 

 ity, which shows that the species may perhaps be maintained. The rather slight difference 

 in absolute dimensions produces a distinct change in habitus which seems to be constant in 

 the specimens. The elongate ovoid outline, relatively narrow base, and more rapid spread of 

 the calyx wall, are plainly shown by the outline restoration of the principal specimen, which 

 although perfect is somewhat flattened (PI. XXXIX, fig. 9c). The basals are not visible in 

 this specimen, and were probably smaller as a rule than in the other species. One fragmen- 

 tary specimen of this species, not figured, has 3 primibrachs in one ray. 



Good specimens of this species are very rare, those figured being all that are known 

 except a few fragments. Several imperfect specimens have been obtained from Button 

 Mould Knob and other localities near Louisville, which furnish useful information as to the 

 structure of the calyx ; some of them have a considerably greater development of basals than 

 is considered characteristic, and in this respect show variation toward M. burling tonensis 

 which is from substantially the same horizon (figs. 14-16). One of these specimens 

 (figs, na-d) is of interest because it shows both the exterior and interior of the base, and 

 gives an excellent idea of the relative proportions and development of these parts. 



As in Ichthyocrinus, the axial canal, which is obtusely pentagonal below, opens out on the 

 inside into a three-cornered funnel with a projecting rim, following the median line of the 

 infrabasals ; this structure at the interior is very well shown by the enlarged figure lie. 

 This interesting specimen, and also the beautiful original of figures ga, b, were given to me 

 some years ago by the Rev. H. Hertzer, of Marietta, Ohio, who collected them at the Knob 

 south of Louisville, Kentucky. Another instructive specimen from the same region gives a 

 vertical section of these structures, obtained by parting at the suture lines, not by grinding 

 (fig. 12) ; the proximal columnal is in place, and the axial canal is seen passing through it and 

 the infrabasals, enlarging above into the chambered organ. In none of these specimens show- 

 ing the interior can I find any trace of grooves for the lodgment of the axial nerve cords 

 after they pass upward beyond the triangular funnel within the infrabasals ; their course was 

 probably entirely separate from the calcareous substance of the plates. 



Types. Miller and Gurley's original is in the University of Chicago ; the other specimens 

 figured are in my collection. 



Horizon and locality. Lower Carboniferous, New Providence shales (Knobstone) ; 

 Clark County, Indiana; Knobs near Louisville, and Bradfordville, Kentucky; White's Creek, 

 Tennessee. 



