740 OX THE YOUNG STAGES OF ECHINI. 



the relative breadth of the ambulacra] and interambulacral zones, are identi- 

 cal in Mell it. i testudinata and Encope emarginata. What is remarkable in 

 Mellita testudinata is that the mode of formation of the ambulacral lunules is 

 not identical with that of M. hexapora. The interambulacral lunule alone 

 (PI. XI. f. i-',, is) is developed from a depression formed on the lower surface 

 pushing its way through the test, while the ambulacra] 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 (PL Xl.f. ig), — 

 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. In fact, the mode of development of Encope and of Mellita testu- 

 dinata, as well as of Mellita longifissa ( /'/. Xl.f. .'; :;). is far more closelx 

 allied than that of the two species of Mellita of the types of hexapora and 

 test udinata. 



In Encope emarginata we have, as in Mellita. an early stage ill which no 

 posterior interambulacral lunule exists | /'/. XII. f. />). The outline of these 

 young Encopidae is not Laganum-like, as in Mellita. hut is elliptical, as in 

 very young Echinarachnius : the ambulacral zones, extending uniformly from 



the edge to the apex, are narrower than the interambulacral (PI. XII. f. 1J,). 

 The plates of both areas carrj one to two Large tubercles and a couple of very 

 small ones. The ambulacra] pores extend from the apex to the mouth. One 

 pair of pores, not connected by grooves, is situated in the suture of each am- 

 bulacra] plate. The outline seen from above is deeply scalloped ( PI. XII. 

 /. /;, /.;). — in fact, it is ;i Moulinsia, — and the figure given by Agassiz in the 

 Monographic des Scutelles is only a young Encope emarginata. The pos- 

 terior interambulacral lunule commences as a pit from the lower side (PI. 

 XII. f. w), and by the time the young Encope has attained a diameter of 

 three quarters of an inch, the lunule is seen from above ( PI XII. J. i;) as ;i 

 small elliptical opening. The edge of the test is deeply scalloped, especially 

 at the median ambulacral sutures, where notches soon appear, and the 

 young Encope gradually takes a deeply lohed outline ( PI. XII. f. 17, 80, .'.'. ..", 

 These cuts may or may not close, and thus we have the basis of the great 

 number of species established upon the depth of lobes, the presence or ab- 

 sence of certain lunules. which are nothing but features of the young either 

 retained in the adult or greatly exaggerated. The ambulacral rosette is 

 formed as in Mellita and in Echinarachnius by the independent growth of the 



