442 



MERISTIC VARIATION. 



[part I. 



the parts missing are those which lie on the left side of a line drawn 

 through the middle of the anterior single ambulacrum and the posterior 



a a , 



1 



> 



_/" 



i~y/ 



<"'V V» '•.■: 



•*'' /,'■ 



Fig. 136. Echinoconus subrotundus having 4 rays, No. 677. (From Roberts, 

 Geol, Mag., 1891.) 



1. View of apical system. 2. Seen from side. 3. From apex. 4. From below. 

 aa, anterior ambulacrum [?]. mp, madreporite. ai, anal interradius. 

 The parts are lettered after Roberts. 



unpaired interambulacrum, but it is not possible to say which of the 

 paired areas of this side are wanting, as the pores in the ambulacral 

 plates round the peristome are indistinctly shewn. Roberts, T., Geol. 

 Mag., 1891, Dec. in., vm. p. 116, figs. 

 678. Discoidea cylindrica : a 4-rayed specimen, absolutely sym- 

 metrical. There are only 4 oculars corresponding with the 4 ambu- 

 lacra. Cotteau, G., Peel, franc., 1862—67, VII. p. 31, PL 1011, figs. 6 

 and 7. [This is exactly like Roberts' case No. 677 and is illustrated 

 by beautiful figures (q.v.). Cotteau in describing it says that the 

 anterior ambulacrum is wanting. It is difficult to see any sufficient 

 reason for the determination that this ambulacrum in particular is 

 wanting. For in this case there are only 4 sets of interambulacral 

 plates as well as 4 ambulacral areas in perfect symmetry. The anus 

 of course lies between two ambulacra ; and as the whole number is 

 even and the radii are symmetrically arranged, there is thus no ambu- 

 lacrum in the plane of the anus. Hence the suggestion that it is the 

 anterior ambulacrum which is wanting. But if by Variation an 

 Echinid has 4 symmetrical radii it would always seem that the 



