H. L. Haiokins — Apical System in the Holectypoida. 13 



E. The Apical System of Holectypus. 



As was the case in Pygaster, this genus is capable of division into 

 two clearly defined groups on the structure of the apical system. 

 These groups Avere named by Pomel (1883) Holectypxis \_sens. str^^ 

 and CoRnholectypxis. Strati graphically they are restricted to Jurassic 

 and Cretaceous forms respectively. ' Solectypus ' has only four genital 

 pores, while ' Coejiholectypus ' has five. This is not, perhaps, such an 

 important distinction as some authors have considered, but, in view of 

 the stratigraphic value of the distinction in this case, I think that 

 Pomel's name Ccenholedypus, clumsy though it is, should be given at 

 least a subgeneric sense for Cretaceous Holedypi. 



1. Solectypus, sens. str. (PL II, Fig. 5.) 



The most striking feature of the apical system in the Jurassic forms 

 is the extreme prominence of the madreporite. This is central in 

 position and dome-shaped. In S. depressiis its prominence is perhaps 

 more noticeable than in other species, but in S. liemisp]i(Bricus (where 

 the whole system is not infrequently sunk below the level of the 

 corona) it is still well marked. The water pores are, as usual, 

 entirely restricted to the right anterior genital, which is therefore, as 

 in Pygaster, sens, str., very largely developed in its central extension. 

 The full complement of five genital plates is present, and the posterior 

 one, although imperforate, is often quite as large as the other three 

 (ordinary) plates. The central development of the madreporite is so 

 great that the oculars, no less than the genitals, are all in contact 

 with it. In S. depressus, indeed, the oculars are almost of the same 

 size as the genitals. 



2. Ccenholectypus. (PL II, Pig. 6.) 



The only constant difference in the apical system between this 

 group and the last is in the perforation of the fifth genital. The size 

 and position of the madreporite are similar in both divisions. 



In Solectypus, sens, lat., then, we see a complete recovery of the 

 apical cycle in shape and in composition, and the monopolization of 

 the centre of the system by the madreporite. 



F. The Apical System of Bisooidea. (PL II, Fig. 7.) 



In the case of Piscoidea there are two variable characters in the 

 structure of the system which seem to be quite casual in their 

 occurrence. Even Pomel was unable to found ' generic ' distinctions 

 upon them ; and, indeed, they may differ in individuals of the same 

 species. The two characters are : the presence or absence of a pore 

 in the fifth genital, and the restriction of the madreporite to its 

 normal plate, or its distribution over all five genitals. (The 

 perforation of the posterior genital is usual, though not invariable.) 

 The irregularity of development of such seemingly important features, 

 within the limits of an otherwise circumscribed genus, would appear 

 to indicate clearly the plasticity of structure which is generally 

 associated with the rapid differentiation of new types. Pisco'idea, as 



