ANATOMY OF THE TEST OF DISCOID EA CTLINDRICA. 



49 



presenting in some species five genital plates whicli are per- 

 forated, and in some other species a complementary imperforate 

 plate in the position of tlie odd generative plate " (Ech. du Dept. 

 de la Sarthe, 1869, Supp. p. 412). Desor had restricted his diagno- 

 sis to the structures with which he was acquainted ; but his speci- 

 mens do not appear to liave been good ones, for he stated that the 

 apical system is small and rarely distinct in its details ; but he 

 noticed that the odd genital plate is not perforated by the duct, 

 and that the ocular plates are very small (' Synopsis,' p. 175). 



The late Dr. AYright is at issue with all the other describers 

 of the species about the extent of the madreporlte ; and his de- 

 scriptions of the ambulacra and interrsidiaj of Discoidea cylindrica, 

 upon which he placed a specific value, are doubtful There are 

 therefore many reasons why this familiar form should be studied, 

 and especially as in one of the many beautiful specimens in the 

 British Museum the perignathic girdle is exposed and can be 

 understood. We propose, therefore, ofi*ering the results of our 

 study of the most important details of the test in this and 

 some other species of the genu8, and we trust that some of 

 the diff'erences of opinion have noAV been settled, and that some 

 fresh anatomical details have been utilized. 



Apical System. — The specimens examined in reference to this 

 and all the other anatomical structures are in the British Museum, 

 the Museum of Practical Geology, Jermyn Street, and one is in our 

 possession. The full-grown specimens show^ how small the apical 

 system is in relation to the dimensions of the test, that tliere are 

 five basals, none being complementary, and that whilst four of 

 them (Nos. 1 to 4) are perforated by genital ducts, all of the five 

 are penetrated by the madreporite. There are faint or decided 

 grooves between the basals. In the largest specimens the distri- 

 bution of the madreporite is well shown ; and in No. 4(J63, Brit. 

 Mus., the fifth basal is large and pentagonal, and is marked only 

 by a few of the pores of the madreporite ; and in a smaller spe- 

 cimen which is half-grown, belonging to us, the pores are absent 

 in the fifth basal. 



Dr. Wright (Monogr. Cret. Echin., Pal. Soc. Lend. 1874, vol. i. 

 pt. vi. p. 207, pi. xlvi.) refers to the apical system. The type he 

 examined has a perfect apical system, and each of the basals is 

 perforated by the madreporite, and four basals are perforated by 

 the genital ducts. But the author, by an unfortunate oversight, 

 states that the madreporite is only seen in the right anterior 



LINN. JOURN. — ZOOLOGY, VOL. XX. 4 



