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



(b) The Bridport norm. (PI. XVI, Fig. 2.) 



Here the long axis of the madreporic genital is inclined at an 

 angle of between 60° and 55° in relation to the antero-posterior axis. 

 The genital plates are subequal, but plate 3 is somewhat reduced. 

 Owing to the obliquely transverse extension of plate 2, plates 3 and 

 5 are widely separated from one another, and plate 4 meets plate 2 

 along a slightly curved suture of some length. The oculars are 

 similar to those of the Broadwindsor type. 



Among the twelve Broadwindsor specimens, and thirty-two Bridport 

 specimens, that conform to Fig. 1, there are, of course, numerous 

 trifling differences in the proportionate sizes of the plates and the 

 angles of the sutures, but none of these differences affect the relation 

 of the plates to one another. The same remark applies to the 4 and 

 101 specimens represented by Fig. 2. The two figures are copies 

 of specimens that show the average characters for the respective 

 tvpes. Since the axis of the madreporic genital of Plesiechinus is 

 almost parallel to the antero-posterior axis, and that of Cotudus 

 considerably inclined, the Bridport norm would seem to be progressive 

 beyond the Broadwindsor type. 



(c) The aberrant forms. 



Beside the two alternative structures in the apical system above 

 noted, there are three classes of variants that show more striking 

 departures from the normal. These classes are illustrated by PI. XVI, 

 Figs. 3, 4, 5, and 6. 



(1) Variant 1. (PI. XVI, Fig. 3 ; cf. also Figs. 6, 7, and 8.) 



This variation, which consists in the transgression- of the madre- 

 porite on to genital 3, is by far the commonest type of abnormality 

 in the series examined, and among other Holectypoids as well. Of 

 the six aberrants from Broadwindsor five possess this character ; one 

 of them (Fig. 6) combining with it a different type of variation. 

 Of the thirty-four aberrants from Bridport, no fewer than thirty-two 

 possess a similar character. The five Broadwindsor specimens are all 

 ''norms" for their locality as regards the relations of the genital 

 plates, but, as a comparison of Figs. 1 and 3 will show, the pro- 

 portions of the plates are various. Thirty of the Bridport specimens 

 have the "Bridport norm" in plate relations, the other two being 

 like those fz*om Broadwindsor. 



I have not seen any specimens of H. hemisph&ricus in which the 

 madreporic perforations extend on to any other plate than genital 3. 

 Variants of this character, often carried to a further degree, are by 

 no means uncommon in Conulus (PI. XVI, Fig. 8). In the specimen 

 figured all four genital plates are perforated by the madreporite, but 

 it is only on plate 3 (of the abnormal ones) that the pores are at all 

 abundant. 



In Discoides the extension of the madreporite on to all five genital 

 plates is a generic character (PI. XVI, Fig. 7). The variants of 

 Holectypm of the type under consideration may then be considered 

 to be progressive in the Biscoides direction, while the similar variants 



