MELLITA TESTUDINATA. 823 



gradually increase in size and become more widely separated, especially near 

 the actinostome from a distance of one quarter of the radius, where they are 

 quite distant. The ambulacra! furrows, sending out a principal branch on 

 each side of the median ambulacra! space, send out short processes over the 

 whole lower surface, and longer ones near the edge of the test; they diminish 

 considerably in number and distinctness upon the interambulacral spaces, 

 especially the posterior space. The median ambulacral zone carries large 

 tubercles, but they disappear near the branches of the poriferous zones, 

 leaving a considerable distance on each side of them apparently bare, but 

 covered in reality with very minute tubercles carrying diminutive spines. 

 The arrangement of the spines on the lower side in the two species of Mellita 

 is peculiar; the large spines of the anterior interambulacral spaces are 

 directed outward, those of the posterior interambulacra are turned inwards. 

 On the upper part of the test the spines all turn towards the periphery. 

 The color when alive is a greenish-blue. 



The general character of the changes undergone by Mellita sexforis, as far 

 as they relate to the transformations of the ambulacral rosette, the growth 

 of the tubercles, the changes in the proportions of the relative breadth 

 of the ambulacral and interambulacral zones, is identical in Mellita testudi- 

 nata and M. longifissa. What is remarkable in Mellita testudinata is that the 

 mode of formation of the ambulacral lunules is not identical with that of M. 

 sexforis. The interambulacral lunule alone is developed from a depression 

 formed on the lower surface pushing its way through the test, while the am- 

 bulacral lunules are the result of the closing in of notches appearing on the 

 edge of the test, which remain open until the young Mellita has attained a 

 considerable size, — three quarters of an inch and sometimes more; long 

 after the arrangement of the plates, the shape of the rosette, the size of the 

 tubercles, and the extent of the poriferous zone on the lower surface have 

 the character of the adult. 



The smallest specimens of Mellita observed, measuring about 3.6 1 "" 1 in 

 diameter, are nearly circular ; they have at this stage but a single lunule, 

 the posterior interambulacral one, scarcely perceptible from above, but 

 from the lower side well seen as a deep conical j>it pushing its way gradu- 

 ally more and more towards the abactinal side, and becoming larger and 

 larger when seen from that side with increasing age ; when it has at- 

 tained a diameter of about S." im , the centre of the ambulacral edge of the 

 test becomes slightly indented, — the first trace of the ambulacral lunules 



