ON AN ABNORMAL AMBLYPNETTSTES. 129 



make any observation, it may be pointed out that the anal plates 

 are hardly to be compared with any part of the corona or of the 

 genital or ocular plates. The anus is, as Prof. Loven tells us *, 

 " produit par une resorption locale de la substance calcaire ;" 

 even if the anal plates have a deeper morphological significance, 

 they are not so constantly four in Echinocidaris as has been 

 ordinarily supposed t. 



The pentamerous arrangement of parts in the regular Echinida 

 is, then, only disturbed heretofore in one example^; information 

 and specimens are, however, at hand to show how this may have 

 happened ; the rarity of any divergence from this five-part dispo- 

 sition, in face of the numerous variations which occur in other 

 Echinodermata, will doubtless become more and more important 

 as a factor in determining the genealogical history of the group. 



The following are the more important measurements of the 

 specimen (in millims.) : — 



Diameter. Height. Abactinal area. Anal area. Actinostome. 

 135 11 3-5 ■ 1 5-5 



The specimen was collected at Station 162 (off East Moncoeur 

 Island; depth 38 to 40 fathoms). 



DESCRIPTION OF PLATE V. 

 Fig. 1. Test, seen from abactinal surface. 



2. Test, from actinal surface. 



3. Apical area. 



The lettering to the above figures applies as follows : — ia, interambulacral, 

 ap, ambulacral plates; m, madreporic plate; o, ocular, g, genital plates. 

 For figs. 4, 5, and 6, see Mr. Stewart's paper, postea. 



* " Etudes sur les Ecbinoides," Kongl. Svensk.Vetensk.-Akad. Handl. ii. no. 7, 

 p. 90. 



t Proc. Zool. Soc. London, 1879, p. 436. 



\ [Mr. Stewart's example of an opposite kind of malformation was not known 

 to me when this paper was written.] 



LINN. JOTJBN. ZOOLOGY, VOL. XV. 10 



