On the P eriproctal Plates of Discoiolea cylindrica. 499 



by Commander Campbell Hepwortli, the wind on this coast 

 varies from S.W. to N.W. We have thus a period during 

 which the drift-currents will accelerate the N.E. going ebb 

 and favour the northward dispersal of the larvae. 



Works Quoted. 



(i) Orton, J. H. Proc. Malacological Soc. xi. 4, 1915, p. 190. 



(z) Mueie, J. ' The Zoologist,' iio. 845, November 1911. 



(3) Orton, J. H. Journ. Marine Biol. Assoc, x. 2, 1914, p. 320. 



(4) Lo Bianco, S. Mittk. Zool. Station zu Neapel, 19 Bd. 4 Hft. 1909, 



p. 527. 



(5) Orton, J. II. Journ. Marine Biol. Assoc, ix. 3, 1912, p. 439. 



(6) Conklin, E. G. Journ. Morphology, xiii. 1897, p. 19. 



(7) Orton, J. H. Trans. Plymouth Inst. xv. 4, 1915, p. 250. 



LX. — On the Periproctal Plates of Discoidea cylindrica 

 {Lamarck). By Frederick J. North, B.Sc, F.G.S., 

 Assistant Keeper, Geological Department, National Mu- 

 seum of Wales. 



The periproct in fossil echinoids belonging to the Exocycloida 

 usually appears as an open hole in the posterior interambu- 

 lacrum, the plates which covered it during life being lost 

 as a result of the decomposition of the membrane which held 

 them together. This is not to be wondered at when the large 

 size (in many genera) and the exposed position of the peri- 

 proct are considered. Occasionally, however, the periproct 

 is seen to be covered by a mosaic of plates surrounding the 

 very small anal aperture ; but this is so rare an occurrence 

 that little or nothing is known concerning the periproctal 

 system of many genera. It therefore seems desirable that 

 any such occurrence should be recorded, especially as the 

 few instances that are known suggest that the system is of 

 taxonomic value. 



The following is an account of the plates as seen in a 

 specimen of Discoidea cylindrica (Lamarck) from the Upper 

 Chalk of Swaffham, Norfolk, and now in the Geological 

 Department of the National Museum of Wales (reg. no. 

 15. 156. G. 8). 



The specimen in question has a small, almost hemispherical 

 test, the shape and proportions of which are indicated by the 

 plan and profile (text-tig., B & A). The periproct, which 

 occupies a position on the adoral surface rather nearer to the 

 ambitus than to the mouth-opening, is oval in shape, rounded 



35* 



