1908.] AN ABNORMAL ECHINUS. 651 



have been taken by a fully developed median. Twenty-nine is also 

 formed of two primaries ; while thirty, complete as regards the 

 number of plates and pore-pairs, possesses an ariungement 

 altogether unusual. The apparent adoral plate is an included 

 plate, for it touches the zigzag suture, but fails to reach the 

 straight suture between ambulacrum and interambulacrum ; the 

 demi-plate is present, but instead of being median, it is external, 

 touching plate twenty-nine and lying in the same latitude as the 

 adoral plate ; the aboral primary is normal. The pore-pairs of 

 the adoral and demi-plates are surrounded by a deep hollow 

 instead of by the usual faintly-marked peripodal groove. In 

 series h of area I the adoral plate of twenty-nine has no pore- 

 pair, but the median and aboral are normal. Plate thirty contains 

 only two single plates, a very deep adoral and an aboral demi- 

 plate. It is succeeded by a solitary demi-plate, the position of 

 the pore-pair of which would indicate that it i-epresents the 

 missing aboral of the preceding plate. 



In area II (text-fig. 139), series a, the twenty-eighth plate is 

 formed of two simple individuals, probably an adoral demi-plate and 

 an aboral primary, but the obscurity of the sutures renders certainty 

 impossible. Plate twenty-nine is a single primary possessing no 

 pore-pair. It is succeeded by a solitary demi-plate, this again 

 being followed by a compound plate containing four elements, the 

 aboral of which lacks a pore-pair. In series 6, twenty consists of 



II. 



Abnosmalities in Ambulacral Areas of Echinus esculenUis. 

 (For explanation of tlie lettering see text-fig. 138.) 



a union of six primitive plates, no suture separating a first normal 

 triad from a second, twenty-eight consists of two primaries, 

 twenty-nine of only one, while between twenty-eight and twenty- 

 nine is wedged in a small insulated poreless individual. Succeeding 

 twenty-nine come two separate and distinct demi-plates, the distal 

 about half the size of the proximal ; and these are followed by 

 thirty, possessing only an adoral primary and an aboral demi-plate. 

 In ambulacrum III (text-fig. 140) plate twenty-nine in series a 

 contains the normal number of plates, but the aboral is faintly 

 marked and is imperforate. The succeeding plate is also a 

 normal triad, but the peripode of the median element contains 

 only one pore, and must in its present condition have been 



