322 MELLITA TESTUDINATA. 



(PI. XI. f. 10), not appearing simultaneously, the changes are limited to the 

 increase in size of the lunules (PL XI. f. //), and of the poriferous am- 

 bulacral zone on the lower side; the outline and general facies, with the 

 exception of the larger size of the tubercles, being that of the adult 

 [PL XL ./: /*). 



Contrary to what was to be expected from analogy, we find in Mellita 

 Stokesii, so closely allied to Mellita sexforis, the lunules developed as cuts 

 along the periphery, exactly as in Encope. and in the two other species of 

 Mellita, so thai the mode of development of the lunules does not seem to have 

 any great physiological value, whether formed by the resorption of the test in 

 the centre of the plates, or by a retardation of their growth at the edge of the 

 test; those species which are most closely allied having diametrically opposite 

 modi's of development of the lunules. In fact, the mode of development of 

 Encope and of Mellita testudinata (and M. longilissa) is far more closely allied 

 than that of the two species of Mellita of the types of sexforis and Stokesii. 



Mellita testudinata 



! Mellita testudinata Klein, 17.il. Nat Disp. Echin. 



PI. XI. f. 13-22; PL XII\ .- PI. AY/'./. 1,2. 



Four lateral ambulacral lunules, in continuation of the median line of 

 ambulacra, and one posterior interambulacral one ; outline more or less 

 rounded anteriorly, truncated posteriorly, greatest width usually across the 

 lateral posterior interambulacral space; the organic apex is slightly eccen- 

 tric anteriorly, the test sloping gradually towards the posterior ednc. but 

 curved towards the anterior edge, which is usually thicker, though we find 

 frequently specimens in which the slope is the same towards both edges; 

 the lunules are all narrow, elongate, the interambulacral one being the 

 largest. The petals are of nearly uniform size, the odd and anterior pair 

 slightly shorter than the posterior pair. Each poriferous zone is broader 

 than the enclosed median ambulacra! space; the poriferous furrows are 

 separated by a single row of tubercles very regularly arranged in a line 

 parallel to the furrows. The tubercles covering the upper surface of the 

 test are extremely uniform in size (except those on the edge of the lunules); 

 tiny are closely packed, and separated by ridges of minute miliary tubercles. 

 The large tubercles which cover the interambulacral space of the lower side 

 have no regular arrangement, any more than those of the upper part of 

 the test ; they are small near the edge of the test, and closely crowded, but 



