CHAP. XVIL] ECHINOIDEA. 445 



(582. Hemiaster batnensis : specimen in which the left posterior 

 ambulacrum is not present, and the ambulacral groove is only indicated 

 by a shallow depression, beyond which there are some rounded pores 

 which continue the ambulacral area beyond the fasciole. The corre- 

 sponding ocular seems to be absent. The test is of normal form, but 

 the median suture of the right posterior interambulacrum is not quite 

 straight. GAUTHIER, M. V., Comptes rendus de VAss. pour I'av. des sci., 

 1885, xm. p. 258, PL vn. fig. 1. 



683. H. batnensis : very similar case of absence of right anterior 

 ambulacrum and corresponding genital and ocular plate, ibid., fig. 3. 



684. Hemiaster sp. : left anterior ambulacrum wanting and is gone 

 without trace. There are only 4 oculars and 3 genitals. In corre- 

 spondence with this variation there is considerable change in symmetry 

 of the test, which is irregular, the anterior and right anterior ambu- 

 lacra being deflected from their normal courses. [See details.] Ibid., 

 figs. 4 and 4 bis. [Here, where there is a clear differentiation between 

 the several ambulacra, it is doubtless possible to affirm that such a 

 definite ambulacrum is missing, for the two interambulacra are left 

 adjacent to each other.] 



685. Echinus sphaera (0. F. Miiller): specimen described in which 

 the left posterior interambulacral series of plates is almost entirely 

 absent. The details of the structure are as follows : the genital plate 

 which stands at the head of the left posterior interambulacrum is 

 reduced in size in all directions ; but the two ocular plates which should 

 be separated by it are somewhat enlarged, bearing several extra tuber- 

 cles, and meet together peripherally to the genital plate. The series of 

 interambulacral plates which should begin from this genital plate are 

 represented by a rudimentary row of small tubercles : the ambulacral 

 systems which are normally separated by these plates are consequently 

 almost contiguous. The rudimentary interambulacral series widens 

 somewhat at a short distance from the apical series and forms a small 

 island of interambulacral structure bearing 4 large tubercles. Beyond 

 this, viz. at a point placed about ^ the distance from the apex to 

 the oral surface, the two ambulacra again unite and are continued as a 

 single ambulacrum of double width. DONITZ, W., Mullens Arch. f. 

 Anat. u. Phys., 1866, p. 406, PL xi. 



(3) Case of total Variation to a 6-rayed form. 



*686. G-alerites albogalerus(P) : a regularly 6-rayed specimen having 

 six symmetrical ambulacra and interambulacra (Fig. 138). MEYER, 

 A. B., Nova Acta Ac. Cces. Leop. Car., xvni. 1836, p. 224, PL xm. 



Fm. 138. Galerites alboyalerus, No. 686. A six-rayed specimen. (After 

 MEYER.) 



