68 ON AN ABNORMAL SEA-URCHIN. 
Similar irregularities, though not so well marked, occur in 
plates 50 and 51 of series 6 of ambulacrum IV (text-fig. 4). In 
this ambulacrum the presence of a fourth pore-pair in plate 50 
Text-figure 4. 
Abnormal plates in series a and 6 of ambulacrum LY. 
¢ 
in series @ and plate 52 in series b again suggests the inter- 
polation of small and obscure elements. No. 52 im series 6 
undoubtedly has an interpolated demi-plate. 
