Herbert L. Hawkins — Studies on the Echinoidea, etc. 251 



abundance, and sometimes threaten the supremacy of the all-pervading 

 Terebratula (Sphceroidothyris) li sphceroidalis" . 



The specimens of K. hemisphcericus are commonly small, averaging 

 2 cm. in diameter, and are remarkably uniform in size. It is easy, 

 therefore, to collect plentiful material for a study of variation, 

 without the complication of differences due to age. For the purposes 

 of the present paper (which is only a preliminary note) specimens 

 from two localities have been used, and no others are referred to, so 

 as to avoid confusion. In a collection in the Manchester Museum 

 there are twenty - two specimens (suitable for study) labelled 

 "Broad Windsor"; all, apparently, from one quarry. In my own 

 collection there were 167 specimens from a layer about one foot thick 

 in a quarry on the side of the main road east of Bridport. Many of 

 the latter series have been broken up or otherwise disposed of in the 

 course of other work upon them, but examples of all observed types 

 of variation have been retained. 



Taking the 189 specimens together, the normal structure of the 

 apical system (occurring in 55'5 per cent) is that represented by 

 PL XYI, Fig. 2. But owing to the disparity in the numbers from the 

 two localities, this combined percentage is misleading. The true 

 relations of the " norms " and " variants" may be more satisfactorily 

 expressed by means of the following table : — 



From this table it appears that there are separate "norms" for 

 the two localities, although the small number of the Broadwindsor 

 specimens is unfortunate. Fifty-five per cent of the Broadwindsor 

 forms have a type of apical system which is found in only 19 percent 

 of those from Bridport. And conversely 61 per cent of the Bridport 

 examples possess a structure found in only 18 per cent of those from 

 Broadwindsor. The percentage of aberrant types from both localities 

 is surprisingly high. 



(a) The Broadwindsor norm. (PL XVI, Fig. 1.) 

 The distinguishing character of this type of apical system depends 

 upon the direction of the long axis of genital 2 (the madreporic 

 plate). This axis is at an angle of between 80° and 75° to the 

 antero-posterior axis of the test. The five genital plates are all 

 subequal, but plate 1 is somewhat reduced. Owing to the direction 

 of elongation of plate 2, plates 3 and 5 meet along a short transverse 

 suture, so that plate 4 is not in contact with plate 2. The ocular 

 plates are of considerable size, and are all approximately similar. 



