1881.] PROF. K. J. BELL ON THE ECHINOMETRID-E. 415 



And, dealing with the restricted genus Echinometra, I have 

 myself been able to make somewhat similar observations in the case 

 of E. oblonga, E. subangularis, and E. lucunter. 



If we put into a tabular form the numbers of pairs of pores in an 

 arc, we find the average adult arrangement to present very considerable 

 variation, thus : — 



Echinometra lucunter has 5 or 4 pairs of pores. 



E. oblonga „ 5 „ ,, 



JE". viridis „ 5 „ ,, 



E. subangularis ,, 6 or 5 „ ,, 



E. macrostoma „ 8, 7, or even 3 pairs of pores. 



E. vanbrunti „ 9, S, or 7 pairs of pores. 



Having already insisted on the fact that only three pairs of pores 

 are to be detected in the youngest plates, or some of the plates, on 

 the test of certain so-called polyporous species, we will insist as 

 much as possible on the " polypority " of these forms by detailing 

 the arrangements which are found in the best-developed part of the 

 adult test'. 



( 1 ) Parasalenia gratiosa .... 3 pairs of pores. 



(2) Echinostrephus molare . . 3 (or 4) „ „ 



(3) Echinometra lucunter .... 4 or 5 „ „ 



(4) E. oblonga 4 or 5 „ „ 



(5) E. subangularis 5 or 6 „ „ 



(6) Splicer echinus granularis .. 4, 5, or 6 ,, ,, 



(7) E. macrostoma 7 or 8 „ ,, 



(8) E. vanbrunti 7, 8, or 9 ,, „ 



(9) Strongylocentrotus bullatus 7 or 8 „ ,, 



(10) S./runciscanus 9 „ „ 



(11) S. albus 10 



(12) Colobocentrotus atratus 12 (ca.) „ „ 



(13) Heterocentrotus trigonarius 1.5 (ca.) „ ,, 



We come, then, to the following results : — 



(1) There is a series of forms which exhibits a gradual increase 

 in the number of primary plates which go to form a secondary 

 plate ; but 



(2) This series always retains indications of secondary plates in 

 which only one primary plate has ceased to grow with the rest. 



When we come to take a general survey of the characters ex- 

 hibited by the pore-plates, we tind the primitive arrangement of the 

 Cidaris passing gradually into the complex secondary plates of 

 Heterocentrotus and Colobocentrotus, which would appear to be the 

 most highly differentiated of the Desmosticha. 



' Bearing in mind the words of Prof. Gegenbaur (Uiiters. zur vergl. Anat. 

 d. Wirbelthiere (1864), i. p. 11(5), ''So wiclitig es ist fiir die Aufstelliing von 

 Unterschiedeu der einzelnen Lebeusfonnen und ihrer Einrichtiing, die ausge- 

 bildetcn fcriigeii Zustanile der Organisation zu Untcrscheidungsobjecten zu 

 nebnien;" and Flower, Introductory Lecture (1870), \>. 37- 



