chap, xvn.] ECHINOIDEA. 441 



about 1 cm. from its termination. The ambulacral groove of (Fig. 134) 

 this abnormal ray divides into two branches at a distance of 2 cm. 

 from the edge of the mouth. One of these branches runs along one of 

 the forks of the ray to its extremity without further complication ; 

 but the other branch, belonging to the second fork, divides again 2 mm. 

 from the first bifurcation, so as to form two tracts which unite with 

 one another 3 mm. further on, thus inclosing a small piece of the ordi- 

 nary integument in an ambulacral area. Finally, this ambulacral area 

 divides once more close to the tip of the ray. There are no signs of 

 injury or disease in the specimen. Herdman, W. A., Nature, 1886, 

 xxxi. p. 596. [I am indebted to Professor Herdman for the accom- 

 panying diagram of this specimen.] 



ECHINOIDEA. 



In the Echinoids there are (1) cases of total Variation to a 

 4-rayed form with 4 ambulacra and 4 interambulacra 1 : (2) cases 

 of partial or total disappearance of a definite ambulacrum or 

 interambulacrum, which can be named either because part of 

 it is present, or because two sets of similar plates thus become 

 adjacent : (3) a case of total variation to a 6-rayed form : (4) cases 

 of imperfect reduplication of a radius, thus forming an imperfectly 

 6-rayed form. 



(1) Total Variation to a 4>-rayed form. 



76. Cidarites coronatus?: 4-rayed regular specimen (Fig. 135). 

 Meyer, A. B., Nova Acta C. L. C, xvm. 1836, p. 289, PL xm. 



Fie. 135. Cidarites coronatus? No. 676, a regularly 4-rayed specimen from 

 oral surface. (From A. B. Meyer.) 



77 Echinoconus (Galerites) subrotundus : 4-rayed specimen in 

 Woodwardian Mus. (Fig. 136). The ambulacral and interambulacral 

 areas are relatively wider than in a normal of the same size, the space 

 of the areas that are wanting being as it were shared among those that 

 are present. Apical disc roughly rectangular, and seems to be com- 

 posed of 4 perforated basals (genitals) and 4 perforated radials (oculars). 

 The basal plate corresponding to the posterior unpaired interambulacral 

 area is perforated, though normally imperforate. Statement made that 



1 Cu£not, Arch, de Biol, 1891, xi. p. 632, says that Echinoconus vulgaris 

 has been seen with only three radii, but no authority is given. 



i 



