292 Prof. S. Loveii on the Structure of the Echinoidea. 



plate ; but sometimes all three are entire ; in series I. h-N. a 

 both primary plates are always entire. 



The order which prevails in the ambulacra at the peristome 

 recurs at the vertex. The corona of a young Toxopneustes 

 drohachensis of 4 millims. diameter, and with a stoma of 2"4 

 millims. is shoAvn by the proportions given in the following- 

 table, which states for each large plate the number of its pri- 

 mary plates, of which the half ones are included in paren- 

 theses : — 



It will be seen that the number of primary plates in each 

 large plate increases from the peristome towards the vertex. 

 In the rows of series I. a-V. b this increase is not quite regular, 

 but somewhat unequal in the large plates 3 and 4, Avhich may 

 be an individual peculiarity ; but the rows I. a-V. h of the 

 bivium agree perfectly, even in the last plates, where the tri- 

 vium also forms a group of similarly developed plates. In 

 series I. b-Y. a, however, the increase is the same throughout. 

 In large plate 3 some individuals have 1 (2) 3. 



In each row the peristomial plate 1 is the oldest ; the other 

 plates are younger in proportion as they are further from this ; 

 and the youngest is that which strikes upon the eye-plate of 

 the vertex. The youngest plate has not the same ordinal 

 number in the rows of both series. In the rows of series I. a- 

 V. b it is the sixth, in those of series I, b-Y. a the seventh, 

 with the, perhaps, individual exception of I. b- so that this series, 

 jvliich in the peristome has one primary plate less than series 

 I. a-Y. b, has in its increasing end one or several more than 

 this, inasmuch as not only is the large plate 6 completed, but 

 even the large plate 7 is commenced. 



Each large plate commences as follows : — Close to the aboral 

 margin of a previously completed large plate the new first 

 primary plate is formed, close to this the second, and so on. 



