Herbert L. Hawkins — Studies on the Echinoidea, etc. 255 



all from the same locality, and those of the remaining six that 

 approximate to the size of this specimen have the characters of 

 Pig. 10. So there are indications that another series of variants 

 could be traced in this species. The original of Fig. 11 cannot he 

 regarded as progressive towards the Corallian species (H. oblongus, 

 Fig. 12), for in six specimens of that form, from Upware, the ocular 

 " insertion " is identical with that of the Cornbrash " norm ". The 

 contact between genital 2 and ocular IV in this specimen suggests 

 comparison with the conditions in the apical system of Conulus 

 (Fig. 8), but that feature, and the large size of the madreporic 

 genital (not a variant character) are the only points of resemblance 

 between the two. 



4. Summary. 

 The characters of the apical system of a series of Holectypus 

 hemisphcericus from the same horizon at two localities in Dorsetshire 

 are analysed and described. It is found that the average relations 

 of the plates of the system are different at the two localities, 

 although certain numbers of identical forms occur at both. Out of 

 189 specimens (from both localities), 40 show serious departures 

 from the normal type. These abnormalities are of three classes. 

 One, the most prevalent, consists in the presence of madreporic pores 

 on genital 3, in addition to the normal perforation of genital 2. 

 This is regarded as a "progressive variant" in the direction of 

 Discoides. The second, occurring in three specimens, consists in the 

 interpolation of a supernumerary plate within the system. It is 

 suggested that this maybe either a "regressive variant" towards 

 Acrosalenia, or a "progressive variant" towards Nucleolites (as 

 illustrated by N. orbicularis). In neither case would this variation 

 coincide with actual phyletic sequence, so that it is styled " parallel 

 variation ". The third type of variant, seen in one specimen only, 

 combines both the first and second types, and in addition shows 

 an absence of genital 5 and a corresponding increase in the size 

 of the posterior oculars, which meet round the back of the system. 

 The variation in this specimen is interpreted as being " progressive " 

 towards Discoides, "parallel progressive" or "regressive" towards 

 Nucleolites or Acrosalenia respectively, and " progressive " towards 

 Conulus. There are indications of a different series of variants in 

 the Holectypus depressus from the Cornbrash. The high percentage 

 of variation in the composition of the apical system of Holectypus is 

 regarded as an indication of the evolutional activity of the genus, 

 and of its near approximation in time and phylogeny to the common 

 origin of many of the groups of Irregular Echinoids. 



EXPLANATION OF PLATE XVI. 



All figures are considerably magnified, and are brought to a uniform size for 

 convenience of comparison. All are viewed from the outside of the test, so 

 that the numbering of the plates is in an anti-clockwise direction. 

 Fig. 



1. Holectypus liemisphcericus. Broadwindsor. The normal apical system 



for this locality. Genitals 3 and 5 in 

 contact. 



2. ,, ,, Bridport. The normal apical system for this 



locality. Genitals 3 and 5 separated by 2. 



