654 MESSRS. RITCHIE AND MciNTOSH OX [June 16, 



iive ambulacral areas, not a single row is free from more or less 

 marked abnormality. Further, all the abnormalities, with two 

 exceptions, are grouped in a band, broken by the interambulacra, 

 which passes round the test at a definite distance from the 

 peristome. The exceptions are the twentieth plate in II h and 

 the thirty-eighth in lY h. 



With regard to the apical disc as a whole there is little worthy 

 ■of note. The plates are normal in number and arrangement, but 

 the whole disc has become slightly elongated as if the part 

 towards the abnormal area had been dragged downwards by it. 

 Consequently several of the genital plates have lost the bilateral 

 symmetry which usually characterises them. The ocular plate 

 corresponding to the abnormal ambulacrum is of unusual shape, 

 possessing four, instead of five, sides and presenting an angular, 

 instead of an almost straight, boundary to the corona. The 

 ocular pore is absent, but its position is probably indicated by a 

 minute pin-hole, which fails to penetrate to the inner surface of 

 the plate, for neithei' is there any sign of an internal opening, 

 nor can a strong light pass through. 



Probable Developmext of the Specimen. 



To bring those observations into relation to one another, the 

 most satisfactory way is to trace the probable development of 

 the shell. It is with the idea of attaining an approximation 

 to chronological sequence that the plates have throughout been 

 reckoned from the peristome, and not from the apical termination 

 of the sei-ies to which they belong. This mode of reckoning 

 has the disadvantage of increasing the difficulty of numerical 

 determination owing to the excessive compression of plates which 

 takes place as the peristomal region is approached, but it has the 

 advantage of following the natural course of development. For 

 it is evident that, since all the coronal plates are foi-med around 

 the margin of the apical disc and are pushed thence down the 

 sides of the test, the oldest plates will lie around the peristome. 

 Thus, counting from the oldest plates recognisable towards those 

 more recently formed, we get a measvire of the age of the animal 

 computed according to a standard, not of time but of develop- 

 mient *. Thus, instead of saying that when a certain plate was 

 formed, the test was three months old, a statement which our 

 ignorance of the growth of the Echinoid imago renders impossible, 

 we can say that at that time the test was, say, ten plates old, the 

 actual age of course being indicated by the formula 10 + a;, where 

 X represents the number of the plates which have been pushed 

 over the edge of the peristome in any one series. But since we 



* Such a measure, it need scarcely be said, is not absolute but comparative, for the 

 iirst plates, and we know not how many of their successors, have already been 

 pushed over the edge of the peristome and are no longer reckonable. Assuming, 

 however, that in each series the rate of pushing over is approximately the same, we 

 an-ive at a measure sufficiently accurate for all practical purposes. 



