500 



Mr. F. J. North on the 



adorally, and acuminated adapically. It is 4 mm. long and 

 2*75 mm. wide. 



The anal aperture, which is rather less than 0*5 mm. in 

 diameter, is situated on the adoral half of the long axis 

 of the periproct, and since the whole system is adapically 

 situated, the anus is just midway between the ambitus and 

 the oral opening. It is raised slightly above the plane of the 



A, 13, C, D. Discoidea cylindrica (Lamarck), Upper Chalk, Swaffham, 

 Norfolk. In (Jeol. Dept., National Museum of Wales. 

 Keg. no. 15. 15G. G. 8. 



A. Profile (p, periproctal system), nat. size. 



B. Plan of adoral surface (m, month-opening), nat, size. 

 0. Periproctal plates, X 12. 



I). Ditto, side view (a, anal aperture), X 12. 



E. Pei'iproctal plates of Discoidea minima, Agassiz, enlarged (after 

 Wright). 



adoral surface of the test, and is surrounded by the peri- 

 proctal plates, which slope from it in all directions, giving 

 the whole system the appearance of a low boss (text-fig., A^ 

 &D). 



The periproctal plates are nineteen in number, arranged 

 in two cycles of nine plates each, with one odd pentagonal 



