FOSSIL CRINOIDS. 151 



POTERIOCRINUS MACROPLEURUS (Hall). 



Plate IV, figs. 7-11. 



1861. Cyalhocrinus macroplenrii.s Hall, Desc. New Crin., 5; Bost. Journ. Nat. Hist., VII, 295. 

 1873. Scaphiocrinus nutcroplciiru.s Meek and Worthen, Geol. Surv. III., V, 412. 

 1879. Vasocrinus macropleiirus Wachsmuth and Springeh, Rev. Pal., I, 96. 



Leaving out the recitals of geometrical shape of the various plates — which 

 made up a tiresome and profitless part of the descriptions of that day, their 

 form for the most part necessarily following from the position and office of the 

 plates — and other immaterial details, the essential part of Hall's description 

 is as follows: — 



"Body subturbinate to the top of the subradial plates (BB), whence it rises 

 more abruptly to the middle of the first radial plates ; abruptly constricted above. 

 First radial plates wider than high, strongly indented above, marked by a medium 

 sized, elevated, subcentral scar for attachment of the arm plates. Surface of 

 plates marked by strong elevations and depressions, forming a series of ridges 

 which originate from the basal plates (IBB), bifurcate on the subradials (BB), 

 and reunite below the scar of the first radial plates; a second set of less strong 

 ridges unites the subradials (BB) across their lateral margins; a third and the 

 strongest set of ridges unites the first radials across their lateral margins. These 

 ridges are formed by the bending or folding of the plates, which are thin, and 

 afford very little substance for their attachment to each other; and this is 

 probably the reason that there are seldom any but detached plates of this species 

 found, while from their frequency it must have been a comparatively abundant 

 species. . . The separated plates are strongly undulated upon their sides, with 

 a very sinuous margin ; these deep sinuosities of the plates in very old specimens 

 show spiculae or bars of calcareous matter extending across the cavity from side 

 to side, thus strengthening the thin joining faces of the plates." 



There is little to add to this description, which might apply equally well to a 

 calyx or detached plate of P. crassus from England or Belgium, or P. pUcalus, 

 as described by the Austins. The arms, if we had them, might show reliable dif- 

 ferences, and it is to be noted that the brachials, as shown by two specimens 

 figured, are extremely short; or it may be that the species actually had such a 

 geographical range, with little change. Hall's description was made from 

 specimens from the Lower Burlington Limestone, and it has not been found 

 in the Upper; it is usually found as detached plates, only one entire calyx having 

 been collected there. Loose plates of it are also numerously found in equivalent 



