1888.] NATURAL SCIENCES OF PHILADELPHIA. 371 



mostly Silurian, of different families, in which the vault is composed 

 of an integument of innumerable minute pieces in Avhich they are 

 undistinguishable, and, in our opinion, do not exist. But we know 

 of no other case of a vault composed of well defined and even 

 ornamented plates, in which in the same genus there was a total 

 absence of plan of arrangement in one species, and well defined 

 summit plates and covering pieces in another. It seemed to us 

 impossible that the summit plates and ambulacra, which were so 

 distinct and conspicuous in the one specimen, should be entirely absent 

 in another species of the same genus ; and the only solution of the 

 mystery which we could arrive at, was that in the latter they must 

 be subtegminal, and that the covering of irregular pieces, shown in 

 the four figures above quoted, was broken away in the specimen 

 which exhibited the summit plates. * 



As we have said before, we had no opportunity to study the 

 Crotalocrinidae from actual specimens when we prepared the 

 Revision. It required but a single glance at the specimens from 

 Dudley and Gothland coming under our observation lately, to show 

 us that our conception of the structure and relations of Crotalocrinus, 

 and its congener, was completely erroneous, and that our views 

 respecting the subtegminal summit plates and double covering are 

 without foundation in the facts. We now renounce them altogether, 

 and all conclusions or arguments based upon the supposed existence 

 of these structures are hereby withdrawn. The same inspection of 

 specimens that disclosed to us our error, revealed with equal clear- 

 ness the real nature of those plates, and left not the least necessity 

 for inferring the existence of summit plates among the irregularly 

 arranged vaults of Angelin's restorations. 



While it is of course unpleasant to be obliged thus to correct 

 descriptions and repair arguments upon which we have laid consider- 

 able stress, we regret it in this case the less, because the result at 

 which we have arrived regarding the systematic position of Crotalo- 

 crinus and Enallocrinus, confirms in a most satisfactory manner the 

 validity of the great groups which we have recognized as subdividing 



* The references to these figures in Part III of the Revision were unfort- 

 unately mixed up in the printing. They should be corrected as follows: on page 

 64, 7th line from bottom, for " PI. 6, " read " PL 8, " and for " figs. 15 and 25, " 

 read figs. 2 and 75 ; " and in the 6th line from bottom, for "PI. 13, " read " PL 

 8. " On p. 65, 6th line from top, for " PI 6, " read " P/. 8. " We also misunder- 

 stood Angelin's fig. 15, PI. 25, and Joh. Miillers' fig. 10, PI. 8, and our references 

 to them should therefore be ignored. 



