ANATOMY OF THE TEST OF DISCOIDEA CYLINDKICA. 
49 
presenting in some species five genital plates which are per- 
forated, and in some other species a complementary imperforate 
plate in the position of the 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. ; hut his speci- 
mens do not appear to have 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. Wright is at issue with all the other describers 
of the species about the extent of the madreporite ; and his de- 
scriptions of the ambulacra and interradia of Discoidea cylindrical 
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, offering the results of our 
study of the most important details of the test in this and 
some other species of the genus, and we trust that some of 
the differences of opinion have now 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 there 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. 4663, 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. Bond. 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. JOUEN. — ZOOLOGY, VOL. XX. 4 
