214 CATALOGUE OF THE BLASTOIDEA. 



neither Hall, Montgomery, Zittel, nor Barris makes any mention of this double series 

 of basal plates ; and we have been equally unable to tind any satisfactory evidence 

 of their presence. There is, however, an impression on the lower face of the basals 

 which corresponds to the upper stem-joint (PI. II. fig. 45) ; but this is nothing more 

 than occurs in a large number of Crinoids, and we see no reason whatever for 

 regarding it as indicating the presence of a ring of " supplemental basals." 



Great misconception appears to have arisen concerning the deltoid plates in this 

 genus. Lyon and Billings supposed them to be only the lanceolate centres of each 

 interradius, " separated from the pieces on either side of the pseudambulacrse by a 

 sharply defined angular ridge, surrounding the whole piece, &c." x These lateral 

 pieces according to their view were attached portions of the ambulacra, and were 

 even supposed by Billings to overlap the central lanceolate portions, the latter being 

 provided with bevelled edges for their reception. It is almost needless to observe 

 that did such " ambulacral plates " exist, as described by Lyon and Billings, we 

 should have an arrangement altogether at variance with anything in the structure of 

 the Blastoidea, as it is at present known to us. Nor does it exist in fact, for we are 

 quite convinced that the four anterior interradii of Elasacrinus are perfectly normal 

 in character (PI. II. figs. 43, 44, 46) ; though we admit the presence of an anal plate 

 as described by Hall. Lyon and Billings were led astray by the diversity of the 

 ornamentation on different parts of the same plate, by the longitudinal surface grooves 

 marking off a middle lanceolate portion of each deltoid, and perhaps also by their 

 preconceived ideas of ambulacral structure. 



The surfaces of the deltoid plates adjoining the ambulacra on each side of the 

 radio-deltoid sinuses are transversely grooved for a short distance (PI. 11. fig. 46 ; 

 PI. XVIII. figs. 17, 18). This portion is the "broad transversely grooved marginal 

 plate" of Billings, and the " piece ornamented with grooves and ridges" of Lyon. 

 It is true that these lateral portions of the deltoids are grooved, and Lyon and 

 Billings are equally correct in saying that each of the grooves and intervening ridges 

 corresponds to a pore and a plate of an ambulacrum. But these structures are 

 nevertheless integral portions of the deltoid plates, and are scarcely apparent except 

 in weathered specimens. Examples of Elasacrinus are frequently found with the 

 radio-deltoid sinuses empty and the component parts of the ambulacra lost. If, 

 therefore, these lateral portions of the deltoids really belonged to the ambulacra, we 

 should expect to find them also absent in such weathered individuals ; but this is by 

 no means the case. Then, again, in well-preserved examples of E. Verneuili the 

 grooves and ridges gradually die out towards the summit ; one portion of the ambu- 

 lacrum, therefore, would be provided with such additional plates, and other parts not. 



The structure of the anal interradius in Elwaer/ims is peculiar and unusual. 

 Lyon described it as occupied by an " anal piece " which is wider than the other four 

 1 Takeout ological Iteport, Owen's 3rd Report Geol. Survey Kentucky, p. 490. 



