1908.] AST ABNORMAL ECHINUS. 647 



a depressed apical region. Closer investigation, however, shows 

 that the general shape has departed considerably from the normal. 

 Viewing the test in plan, one sees marked divei-gence from the 

 apparent radial symmetry chai^acteristic of regular Sea-TJrchins. 

 This is due to a distinct bulging on the side remote from the 

 madreporite, which has caused the ambitus to assume a bilaterally 

 symmetrical, almost oval shape (PI. XXXIII. fig. 1). The same 

 portion, moreover, viewed in elevation, is seen to be considerably 

 depressed as compared with the globular form which charac- 

 terises the other regions (PI. XXXIII. fig. 2). Further, it is to be 

 noted that the apical disc has departed from its normal horizontal 

 position, the madreporite standing at a distinctly higher level 

 than the plates on the opposite side of the periproct, for these 

 appear to have been dragged downwards towards the bulging- 

 portion of the test. On the oral surface the peristomal opening 

 is excentric, it too apparently having been dragged towards the 

 bulging portion, for in that region the mai'gin of the opening is 

 only 29 mm. distant from the ambitus, whereas on the opposite 

 side the distance is 34 mm. 



All those deviations from radial symmetry are due to, or at 

 least are connected with, the fact that a portion of one of the 

 ambulacra is absent. Orienting the specimen in the recognised 

 manner, by placing the aboral surface upwards, with the madre- 

 porite in the right anterior position, and adopting Loven's 

 notation, we find that the incomplete area is number V, the left 

 posterior ambulacriuxi, the tube-foot area of the left division of 

 the bivium. On the aboral surface this radial area is absent, but 

 commencing a little above the ambitus, at the bulging portion of 

 the test, it runs thence to the peristome, being fully repr-esented 

 on the oral surface. 



The most noticeable result of this partial cutting out of the 

 ambulacrum, apart from the general distortion of the whole 

 skeleton already described, is that the two sets of interambulacral 

 plates (areas 4 and 5) come together, four rows of interambulacral 

 plates thus occurring in close proximity. The two sets are 

 separated by a zigzag suture, whereas the edges of interambu- 

 lacral plates abutting against an ambulacral area (that is, the 

 edges corresponding to those bounding the above zigzag suture) are 

 normally straight. Less noticeable are such minor distortions 

 as the divergence of the line of bilateral symmetry, which passes 

 through the middle of areas III and 5, from its normal straightness, 

 owing to a bending of the interambulacral suture in the latter area 

 towards the locality of disturbance. Quite distinct as this devia- 

 tion is on the aboral surface, on the oral surface it does not exist, 

 the line through areas III and 5 being there perfectly straight. 

 Again, abnormal distortion occurs in the interambulacral sutures 

 of areas 4 and 5, and in the junction between areas 5 and I., all of 

 these lines bending with gentle curves inwards towards the point 

 where the ambulacrum has disappeared. These curvatures are 

 obviously due to an increase in the size of the plates in the 



