THE BRITISH SECONDARY ROCKS. 39 



cavity of the piece or the " heart-pit " (Schliiter), the " nutritive 

 canal" of Goldfuss and Geinitz*. From these chambers there rises 

 up the *' axial prolongation" (Dr. Carpenter) or " dorso- ventral 

 vascular axis " (Ludwig) on its way to enter the visceral mass and 

 to join the oral vascular ring. The above-mentioned opening may 

 therefore be termed the " axial opening." 



Now I entirely fail to see why the presence of a simple round 

 hole on the under surface of the centrodorsal of A. italica, instead of 

 the ordinary stellate perforation, should determine the roundness of 

 the upper axial opening. In the first place, the expanded cavity 

 (nutritive canal) lodging the chambered organ is not necessarily 

 lobate, like the perforation in its floor ; and, secondly, the shape of 

 the axial opening is not necessarily that of the nutritive canal into 

 which it leads. Thus in A. rosacea the centrodorsal cavity has a 

 rounded pentagonal or decagonal shape, but its upper opening is 

 frequently constricted and rendered lobate (more so than in any 

 fossil species) by the five projections of its lip that lodge the radial 

 pits. Further, even if there be a round axial opening in A. italica, 

 it does not necessarily follow that the radial pits are absent as 

 Schliiter supposes. Neither does the presence of a stellate opening 

 (now closed internally) on the lower surface of the centrodorsal of 

 A. lenticidaris necessarily involve the presence of radial pits on its 

 upper surface. In the latter case Schliiter's argument would seem 

 to be as follows : — 



1. Because there was a stellate dorsal opening there was a 

 " lobate nutritive canal " (ventral). 



2. Because there was a " lobate nutritive canal " there were radial 

 pits around its border. 



The first proposition is parallel to that already considered with 

 reference to A. italica, like which it does not appear to me to be 

 necessarily true. The chief difliculty which I feel about accepting 

 it is the great variability in the shape of the axial opening of A, 

 rosacea and A. ceJtica, which may be circular, pentagonal, decagonal, 

 as in Ant. essenensis and Ant. tourticef, or more markedly 5-lobed. 

 The stellate dorsal opening of these forms is obliterated so very 

 early, that it would be hardly fair to base any argument upon its 

 shape. But in Glenotremites (Anteclon) j)aradoccus the stellate dorsal 

 opening persisted through life as in some other fossil species. We 

 do not find, however, that there was always an axial opening of 

 corresponding shape. Thus in one of the specimens figured by 



* K'either of these names is a good one. The latter is a relic of the days of 

 Miller and Groldfuss, when the stem was believed to contain a prolongation of 

 the alimentary canal ; while the former is based on the erroneous idea that the 

 chambered organ is a heart. The second name is at any rate the more preferable 

 of the two ; for the stem does enclose a vascular axis proceeding downwards 

 from the chambered organ, which is an important part of the blood -vascular 

 system. 



t The terms decagonal, lobate, and five-lobed appear to be used indifferently 

 by Schliiter. Thus he places A. tourti(B, wliich he describes as having a deca- 

 gonal opening, in the group " ComatulcB with radial pits and a five-lobed 

 opening." 



