AN ABNORMAL SEA-URCHIN. ' 67 
represent one ordinary plate. The presence of a fourth pore- 
pair in this and the preceding plate is difficult to account. for, 
except on the assumption that both represent small interpolated 
elements. 
Text-figure 2. 
Abnormal plates in series a of ambulacrum II. 
For explanation of lettering, see text-fig. 1. 
In series 6 of ambulacrum III (text-fig. 3) plates 49 and 50, 
while of normal composition, are exceptional in that the pore- 
Text-figure 3. 
Abnormal plates in series & of ambulacrum ILI. 
pairs of their respective demi-plates are almost completely 
outside the limits of the plate, and perforate the adjoiming 
interambulacral plate. 
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