Periprocial Plates of Discoidea cylindrica. 501 



plate adjacent to the anus. The plates of" tlie outer ring are 

 larger than those of the inner ring, the Lirgest being the 

 unpaired lozenge-shaped })Iate at the adapical end of the 

 system. Tlie unpaired ))late of the inner cycle, which 

 resembles in shape, but is smaller than, the unpaired jdate 

 of the outer cycle, is situated adorally. None oJf the plates 

 bear primary tubercles, but miliary granules occur on most 

 of them ; probably all were originally so granulated, the 

 granules, which are necessarily very small, having been 

 obliterated during the cleaning of the specimen. 



The periproctal system of the specimen under consideration 

 differs in certain important respects from that of Discoidea 

 minima, Agassiz, which was described and figured by 

 Cotteau *, and copied by Wright f ; it also does not conform 

 to the generalized account for tiie genus Discoidea given by 

 Mr. Hawkins J ; this is, however, accounted for by the fact 

 that, at the time of writing, that author had not had the 

 opportunity of examining the periproctals of 1). cylindrica. 

 From D. minima the specimen described differs in having 

 a larger number of periproctal plates, none of which bear 

 tubercles, and in the absence of any conspicuously large 

 plate corresponding with the adoral plate of D. minima. In 

 the latter the outer cycle has eight and the inner cycle only 

 two plates, nearly half the total area of the periproet being 

 occuj^ed by one plate. Further, the anus is situated ad- 

 apically, instead of adorally as in D, cylindrica. 



No specimen of D. cylindrica of the elevated hemispherico- 

 cjlindrical type, in which the periproctals are preserved, has 

 come under the notice of the writer, but, taking the specimen 

 here described as representative of the large Discoidere, and 

 D. minima as representative of the series of small species, 

 including D. suhucidus (Klein), the nature of the periproctal 

 mosaic seems to be a further item of evidence favouring the 

 separation of the two series. 



Other genera in which the peiiproclal plates bear some 

 resemblance to those of Discoidea are Coenholectypus and 

 Echinoneus, both of which, however, iiave more plates and do 

 not possess the bilateral symmetry characteristic of Discoidea. 



* Cotteau, Pal. Fran9aise, torn. vii. (1866) pi. 1012. 

 t T. Wright, "Cretaceous Echinodermata," Mon. Pal. Soc. vol. i. 

 (1873) p. 219, pi. xlvii. fig. 4 h. 



X H. L. Hawkins, Proc, Zool. Soc. 1912, p. 4G7. 



