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



the structures observed in variant 2, I now incline to the belief that 

 the included plate is a supernumerary, and that genital 5 is wanting. 

 If this is a correct interpretation of the specimen, it would prove to 

 be similar to variant 2 in this respect as compared with Acrosalenia 

 or " Nucleolites" . On comparing oculars I and V in Figs. 6 and 9, 

 a certain resemblance in their distorted shape and increased length is 

 seen. In this respect also the Holectypus variant can be considered 

 as "progressively parallel " to the " Nucleolites " norm. 



A comparison between Figs. 6 and 8 ( Conuhis) is perhaps more 

 satisfactory. The generic character of the Conuhis apical system is 

 the absence of the tifth genital and the consequent meeting of the 

 posterior oculars, I and V. It is seen that in both figures these 

 oculars are much enlarged. Although in the Holectypus, ocular I is 

 the larger of the two, while the reverse is the case in the Comdus, 

 the discrepancy is more apparent than real. Other specimens of 

 Conuhis (both C. subrotundus and C. albogalerus) have ocular I larger 

 than ocular V. The less usual type was chosen for figuring because 

 of its possession of the characters of variant 1 . It will be readily 

 seen that if the supernumerary plate in the Holectypus were removed 

 the consequent readjustments would bring genitals 1 and 4 into 

 contact (as they are in Conuhis); and a very slight alteration in the 

 dimensions of genital 2 would produce an almost exact similarity 

 between the two systems. When it is realized that in Holectypus 

 depressus (see Fig. 11) genital 2 maybe in contact with ocular IV 

 (as is the case in Conuhis), the correspondence between the two 

 structures becomes more obvious. 



Thus while the included supernumerary plate and the posterior 

 oculars (in the variant illustrated in Fig. 6) show an inclination 

 towards the Nucleolites character, the absence of genital 5 and the 

 meeting of oculars I and V indicate a very definite " progressive " 

 variation towards Conuhis. Whatever may be the ancestry of the 

 last-named genus, its line of descent cannot have been remote from 

 that of Holectypus. 



3. Holectypus depressus (Cornbrash). (PI. XVI, Figs. 10 and 11.) 

 Although I have not enough specimens of this large and abundant 

 species (which is surely specifically distinct from the small Inferior 

 Oolite form of the same name), there are two types of apical system 

 shown in the few examples at my disposal. The rnadreporic genital 

 (2) is always large in this species, and the madreporite occupies 

 a prominent position in the centre of the system. In six specimens, 

 ranging in diameter from 2 to 5 cm., the plates are arranged as in 

 Fig. 10. Using a terminology analogous to that employed by 

 Jackson for Regular Echinoids, this type may be said to have 

 oculars I, II, and III " insert " (to the madreporite instead of to the 

 periproct), and oculars IV and V " exsert ". (By a similar argument 

 H. hemisphcericus has only oculars II and III "insert ".) But in 

 one medium-sized specimen (Fig. 11), with a diameter of about 

 3-5 cm., genital 2 is so enormously expanded that the remaining 

 genitals are much reduced in size, and all five oculars are "insert ", 

 being in contact with the rnadreporic plate. The specimens are 



