SPECIMEN OE AMBLYPKETTSTES. 127 



itself even to the touch ; when the spines, light green at their 

 base, were removed, I saw, what indeed I had long hoped to see, 

 evidence that even among the regular Echinoidea circumstances 

 may obtain which lead to the incomplete development of that 

 pentamerous arrangement of parts which is the general rule 

 among the Echinodermata. Just, however, as in Dr. Philippi's 

 Echinus onelo, indications of the fifth segment can be observed 

 on the actinal surface, though they are not so well marked as in 

 that form, for there is, apparently, no representative left of the 

 interambulacral series, and there is not so large a number of 

 ambulacral pores. 



Adopting the ordinary mode of orientation of the test, or, in 

 other words, regarding the madreporic plate as being placed in the 

 right anterior interambulacrum*, we find that the abactinal region 

 has been pushed backwards, and that it is some of the parts on 

 the left side of an axis drawn through the median ambulacrum 

 anteriorly and the median interambulacrum posteriorly that have 

 undergone loss. Closer examination reveals the fact that it is 

 here, just as in Echinus melo, the left anterior segment or area 

 which has thus suffered : the actinostome has been pushed for- 

 wards and to the left. Turning now to the abactinal region, we 

 find that it is composed of ten plates. This is especially interest- 

 ing, inasmuch as Philippi's specimen presented a tetramerous 

 arrangement of the plates of the abactinal area The two genital 

 plates in the modified area are small, the ocular between them has 

 become considerably enlarged, is obtusely triangular, and has its 

 apex directed downwards. About nine plates down the side of the 

 test the characters of the bare median space alter in character : 

 there is a moderately sized and then a large tubercle ; about 

 halfway between these and the regular row of primary tubercles 

 there is, on either side, a sutural line ; and the two lines unite 

 above the just-mentioned moderate tubercle ; so that, as it seems, 

 a wedge-shaped piece is intercalated into the side of the test ; 

 and we have first a large single plate, and then, as is shown by 

 the presence of three primary tubercles on either side, there 

 are three pairs of coronal plates, while on either side of these 

 there are seven pairs of ambulacral pores. 



Erom this description it should be apparent that at a compara- 



* The magnificent researches of Loven confirm the results of earlier Echi- 

 nologists. "Etudes sur les Echinoides," Kongl. Svenska Vetenskaps-Akad. 

 Hand. Band. ii. no. 7 (1872). 



