THE STEM AM) 0AL1 \. 23 



close parallel in Cyathocrinus ; and the resemblance is further increased by the 

 presence in both forms of the large rhomboidal interradial plates, which roof in the 

 greater part of the visceral cavity, leaving only the mouth in the centre, and in 

 Codaster support the proximal ends of the hydrospires. 



In other species of Codaster the ventral edge of each radial is not so nearly 

 horizontal as in ft trilobatus, but V-shaped with the point downwards; and the 

 consequence is that the ventral surface is no longer flat or nearly so, but marked by 

 five more or less depressed areas, the middle and lowest parts of which are occupied 

 by the ambulacra (PI. X. figs. 19, 20; PL XII. figs. 1-6). Each of these areas is 

 known as a radial sinus. Its outer edge is formed by the two prongs of the v, 

 which rise from the radial lip towards the ventral end of the interradial suture. 

 From the point where these three lines meet a more or less well-defined crest 

 extends towards the mouth and marks the middle line of each interradial plate, the 

 boundaries of which descend the sloping sides of the radial sinuses till they meet the 

 ambulacra (PL X. figs. 18, 19 ; PI. XII. figs. 1, 4 ; PI. XVI. lig. 1), just as the 

 radio-deltoid sutures do on the flat top of Codaster trilobatus (PI. XIII. figs. 1, 4, 8). 

 The sides of the radial sinuses are thus formed partly by the ventral surface of the 

 radials, and partly by the interradials. In the flat-topped Codaster trilobatus, the 

 middle line of the interradial stands out rather prominently as a subprismatic, 

 lanceolate ridge, somewhat above the general plane of the hydrospire-slits (PI. XIII. 

 figs. 1, 4, 0, 8-12). But in proportion as the radial sinus gets deeper and 

 deeper, the ridges at its sides become sharper and sharper and appear less and less 

 distinct in the general .aspect of the summit (PL X. figs. 19, 20 ; PI. XII. figs. 1-6). 

 Since they all converge on the mouth they may be conveniently designated as the 

 oral crests or oral ridges. 



When we first proposed this name for the ridge on the middle line of the inter- 

 radial or deltoid piece we, like Wachsmuth and Springer, regarded the latter as 

 representing an oral plate of a Neocrinoid. Now, however, that this view lias been 

 abandoned, Wachsmuth l says, " It is self-evident that the oral ridge of Etheridge 

 and Carpenter should be called the interradial ridge." This term, however, is 

 equally applicable to any ridge occupying an interradial position on the exterior of 

 the calyx, e. </. the medio-dorsal ridge of a basal ; and we therefore think it inex- 

 pedient to limit its use in the sense suggested by Wachsmuth. " Oral ridge " has 

 the double advantage of brevity and of clearly indicating that the actinal and not 

 the abactinal surface is being described ; while we do not see that it involves any 

 question of the homology of the parts concerned. 



When the upper edge of the radial becomes v-shaped, as in Codaster alternatus, var. 

 elongatus, Wachsm., C, pyramidatus, and ft Hindei (PL X. fig. 20 ; PL XII. figs. 2, 5), 



1 Proc. Davenport Acad. Nat. Sci. 1885, vol. iv. p. 77, note. 



