PtErOET ON THE CEINOIDEA. 43 



interraclial area is wider than the other four " with a conspicuous row of decidedly 

 larger and more prominent pieces along the median part," Billings ^ gave a good 

 fio-ure of this in Reteocnnus stellaris, and spoke of it as follows : — " If this series of 

 joints constitute a true arm there must be six arms in this species." Miller, who has 

 examined the original specimen of Rcteocrinus stellaris, describes it thus — "Azygous 

 interraclial area covered by a large number of plates, probably one hundred or more, 

 very unequal in size, the middle row being decidedly larger and more prominent than 

 the others, so as to form a ridge up the middle. The plates in this row, however, 

 do not rapidly diminish in size and fade out in their distinctive character before reaching 

 the top of the vault ; on the contrary, they are longer than the primary radials, four of 

 them reach nearly as high as the secondary radials, and while the specimen is not 

 preserved above this, enough is disclosed to the palseoutologist to show that this series 

 continued up the face of a proboscis that extended, may be as far, or farther, than the 

 arms and the pinnules." 



In default, however, of further evidence I prefer to believe that the middle row of 

 plates in the anal area of Reteocrinus stellaris was of the same nature as, though perhaps 

 on a larger scale than, that of Reteocrinus nealli, which Miller describes as follows : — 

 " Azygous interradial area covered by fifty or sixty plates, very unequal in size, the 

 middle row being decidedly larger and more prominent than the others, so as to form a 

 ridge up the middle, while the other smaller and less prominent ones are crowded in, 

 irregularly, on each side. The plates in this middle row, however, have no uniformity in 

 size or shape ; the first one is large and elongated, the fourth is small and subquadrate ; 

 and the row has become almost obsolete at the sixth plate, where all are nearly of the 

 same size and scarcely distinguishable from the minute pieces which cover the flattened 

 vault, and with which they unite." The figures of this type in the Palaeontology of 

 Ohio (vol. i. pi. ii. figs. 3b, 3c) illustrate this description admirably, the original 

 specimens having doubtless been seen by Miller; while the figure of Reteocrinus 

 subglohosus on the same plate (fig. 2c) shows the incorporation into the body of a 

 pinnule borne by one of the secondary radials. This pinnule is closely surrounded by 

 the minute interradial plates, but may be distinguished from them at its origin just as 

 the anal appendage is. This condition is still better shown va. Reteocrinus richardsoni, 

 Wetherby, which has two " fixed pinnules " in the anal interradius, one on either side of 

 the median appendage. All three are " soldered " together by the minute irregular plates 

 which pass insensibly upwards into those of the so-called "vault;" and the ordinary 

 pinnules on the lower parts of the arms after the last axillary are united in just the same 

 way.' This condition recurs constantly in the Liassic Extracrinus and in the recent 



1 Decades of the Geological Survey of Canaila, vol. iv. p. 64, pi. ix. fig. 4a. 



2 Descriptions of New CrinoiJs from the Cincinnati Group of the Lower SUiirian and the Subcarboniferous of 

 Kentuek}-, Journ. Cine. Soc. Nat. Hist., vol. ii. pi. x^•i. ligs. 1, la. 



