10 CATALOGUE OF THE BLASTOIDEA. 



the basals in the two latter forms is so great that it is difficult to see where the stem 

 could have been attached ; while in good specimens of Astrocrinus the doi - sal surface 

 is uniformly covered with the minute tooth-like tubercles which seem to have borne 

 spines, and there is no sign of any facet, even the sutures of the basals being hard 

 to distinguish (PI. XX. figs. 10, 13). It is, of course, quite possible that this may 

 be a secondary condition comparable to that of the adult Edriocrinus, Agassizocrinus, 

 and perhaps of Marsupites also. Whatever be the case in the latter type, the 

 gradual disappearance of the stem-facet and closure of the cup below has been clearly 

 traced in the two Palaeozoic genera ; and a somewhat similar transformation takes 

 place in the centro-dorsals of many recent Comatula?. The central opening on the 

 dorsal surface is gradually closed up, and the cirrus-sockets obliterated by secondary 

 fleposits of limestone, so that the adult Comatula shows no trace whatever of ever 

 having been attached to a stem. It is just possible that something of this kind may 

 have taken place in Eleutherocrinus and Astrocrinus ; but the two genera are so 

 rare, that the discovery of transitional stages such as are known for Edriocrinus 

 and Agassizocrinus is likely to progress but slowly. 



B. The Calyx. 



Excluding the summit-plates and the various structures which go to form the 

 ambulacra, we may regard the calyx of all Blastoids as composed of three sets of 

 plates, which correspond respectively to the basals, radials, and the primary or calyx- 

 interradials of a Crinoid, more especially of such a form as Cyathocrinus. The inter- 

 radial plates in the Blastoid calyx, to which Eoemer gave the name of deltoids, corre- 

 spond to the plates which were described by Wachsmuth and Springer and by Zittel 

 as the orals of Cyathocrinus, since they were believed by these writers, and also by 

 ourselves, to represent the oral plates of the Neocrinoidea. The position of the ambu- 

 lacra upon the united edges of these interradial plates of Cyathocrinus and the 

 Blastoids always seemed to us to be a difficulty in the way of regarding them as orals, 

 as was first done by Wachsmuth and Springer and by Zittel, though we afterwards 

 adopted their views. Mr. Wachsmuth l , however, subsequently come to the con- 

 clusion, in which we entirely agree, that this homology is not a valid one, and that 

 the so-called orals of Cyathocrinus and the deltoids of the Blastoidea are really true 

 calyx-interradials like those of Platycrinus, Phodocrinus, and a few Neocrinoids 

 (Guettardicrinus and Thaumatocrinus). We shall therefore speak of them by 

 Roemer's name " deltoids," postponing to a later chapter the discussion of the 

 question what plates (if any) in a Blastoid do correspond to the orals of Neocrinoids. 

 Regarding the calyx as a whole, we would first notice two points concerning it, 

 viz. its shape and the relative proportions of its component plates. In several species 

 of Granatocrintis, Mesohlastus, and Pentrcmites the general outline of the calyx is 

 somewhat in the shape of a bell, with the ambulacra extending downwards from the 

 1 Proc. Davenport Acad. Nat. Sci. 1S85, vol. iv. p. 77, note. 



