Herbert L. Hawkins — Studies on the Echinoidea. 393 



the "lappets" of the processes, and indeed the whole ossicles, are 

 shown to overlap the poriferous zones of the ambulacra. The central 

 portion, or true ridge, of each section of the girdle is divided into 

 two main sections by a more or less median suture, and at the margin 

 of the peristome there are three small, unequal and irregularly 

 shaped plates. In two of the areas, Nos. 4 and 5, Loven, with his 

 customary faithfulness to observation, has indicated some of the 

 sutures in the ridges by broken lines, thus proving that in the other 

 areas he could clearly distinguish the outlines of the plates. (No 

 sutures whatever, either in the girdle or on the adoral surface of the 

 test, are drawn in his figure of the "forma elatior", which was 

 evidently a fully grown specimen in which secondary thickening 

 had covered the original surfaces of the plates.) In one of the 

 many preparations of the girdle of Discoides that I have made, there 

 is a distinct median suture in some of the ridges, and fairly con- 

 vincing evidence of the existence of a small triangular plate in the 

 middle line at the peristomial border. By means of thin sections 

 examined under crossed nicols, I have been able to confirm the 

 presence of this small plate. I have not succeeded in tracing the two 

 small plates which Loven figures on each side of the median one ; 

 but the fidelity of his observations is so uniform that there can be 

 no reasonable doubt of their occurrence, in some examples, at least. 

 Loven's drawing differs from that of Duncan & Sladen in these 

 two fundamental points only : it is a faithful copy, not a diagram, 

 and it contains a record of more complete observation. In its 

 essential features I can confirm its accuracy from original observa- 

 tion. It is perhaps worth remarking that I made sketches which 

 agree with Loven's figure (with the exception noted above) before 

 I had adequately studied his drawing. In the detection of sutures, 

 particularly where they are faint and of unusual distribution, it is 

 distressingly easy to be influenced by preconceived ideas. On 

 re-examining my specimens after becoming familiar with Loven's 

 rendering, not only do the sutures previously detected seem far more 

 obvious than before, but it is quite possible to imagine the presence 

 of the two small plates on either side of the median one at the 

 peristomial margin. However, experience has made me so wary in 

 these matters that I have failed to convince myself of the certain 

 occurrence of the sutures which would be the boundaries of such 

 plates. That they were patent in Loven's specimen I fully believe. 



(e) A suggested new interpretation. (PI. XXV, Figs. 8 and 6.) 



In spite of the doubts entertained by Duncan & Sladen, there is 

 no difficulty in recognizing the processes in the girdle of Discoides. 

 Separated from the ridges, they present a marked similarity to 

 those of Plesiechinus (see part iv of this series). It is in the ridges, 

 which were hardly developed in the Jurassic Holectypoida, that 

 the peculiarity occurs. The primordial interambulacral plate is 

 generously represented at the peristomial margin in all five areas in 

 Discoides (see Loven, 1872, pi. xiv, fig. 125). In the majority of 

 those Diademoida which have a strongly developed ridge, this 



