Mackintosh — On a Malformed Corona of Echinus. 207 



istic granulation; plate II. is very much elongated anteroposteriorly, and 

 the orifice for the genital duct has disappeared ; plate III. presents 

 the reverse condition, being broader than deep, and has its genital 

 aperture ; plates IV. and V. are but slightly altered. The ocular plates 

 are scarcely changed in form, with the exception of plate 1, which is 

 considerably elongated at the lower left-hand comer. The anal sys- 

 tem corresponds in outline to the abactinal, being di'awn out from 

 before backwards. 



In the corona the alterations are mainly confined to ambulacrals 

 1 and 2, and to the corresponding interradia, and of these, interambu- 

 lacrtmi I <? is scarcely changed, being merely curved to the right 

 a little more than usual ; I 5 is a good deal altered, its fourth plate 

 (from the top) seems to be made up of two fused together ; and the 

 twelfth and succeeding plates are very much elongated towards the 

 left side, and seem to include the greater part of the actinal boss. 

 Ambulacrum 1 seems to be only represented by the short series of pores 

 seen in Plate 22, fig. 4, which terminates almost at the summit of 

 the boss, and the perforations of which have no very definite arrange- 

 ment. This ambulacral area is thus altogether included in I 3. The 

 next ambulacrum is a curious one. It begins normally, both in 

 arrangement and position, but soon bifurcates, one division going up 

 to the abactinal system to form ambulacrum 2, of which 2 3 is per- 

 fectly ujLaltered, but 2 « is remarkable for the exceedingly scattered 

 disposition of its pores (Plate 21, fig. 2), some of which are placed 

 well within interambulacmm II I, whilst others are situated in the 

 middle line of their own radium. The other half of this ambulacrum 

 runs up to a short distance above the ambitus (equator of the corona), 

 where it abraptly terminates, thus cii'cum scribing interambulacmm 

 II., which, ending in a point a little below the ambitus, fails to reach 

 the actinal opening, and assumes a striking petaloid appearance. 

 There are thus but four ambulacral areas reaching the abactinal 

 system, I^o. 1 being deficient, and its place occupied by the remark- 

 able congeries of plates shown in the figures. 



The dimensions of the corona are as follows : — 



Eound the ambitus , 15 inches. 



Dextro-sinistral circumference, ISf ,, 



Antero-posterior circumference, .... 13 ., 

 Depth (from abactinal to actinal orifice), . . 3i^ ,, 

 As regards the cause of this curious malformation it is difficult to 

 speak absolutely. The altered side of the corona, though presenting 

 more or less of an undulating appearance, has no sharp depressions 

 such as might be caused by fi-acture and repair ; if this has taken 

 place it has been unusually evenly done, for it is seldom that the 

 calcareous parts of Echinoids are repaired without shovring veiy mani- 

 fest tokens of the change. The flattening of the altered side, and 

 the presence of the actinal boss, suggest the idea that the lateral 

 growth of the test was limited by its surroundings, and had to take 



