MUSEUM OF COMPARATIVE ZOOLOGY. 289 



The general character of the changes undergone by Echinarachnius 

 and Mellita hexapora, as far as they relate to the transformations of the 

 ambulacral rosette, the growth of the tubercles, the changes in the pro- 

 portions of the relative breadth of the ambulacral and interambulacral 

 zones, are identical in Mellita testudinata and Encope emarginata. 

 What is remarkable in Mellita testudinata is that the mode of forma- 

 tion of the ambulacral lunules is not identical with that of M. hexapora. 

 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 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, In fact, the mode of development of 

 Encope and of Mellita testudinata (also M. longifissa) are far more 

 closely allied than that of the two species of Mellita of the types of 

 hexapora and testudinata. 



In Encope emarginata we have, as in Mellita, an early stage in which 

 no posterior interambulacral lunule exists. The outline of these young 

 Encopidre is not Laganum-like, as in Mellita, but is elliptical, as in 

 very young Echinarachnius ; the ambulacral zones extending uniformly 

 from edge to apex, are narrower than the interambulacral. The plates 

 of both areas carry one to two large tubercles and a couple of very small 

 ones. The ambulacral pores extend from the apex to the mouth. One 

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

 ambulacral plate. The outline seen from above is deeply scalloped — in 

 fact, it is a Moulinsia, — and the figure given by Agassiz in the Mono- 

 graphic des Scutelles is only a young Encope emarginata. The posterior 

 interambulacral lunule commences as a pit from the lower side, and by 

 the time the young Encope has attained a diameter of three quarters of 

 an inch, the lunule is seen from above, also as a small elliptical opening. 

 The edge of the test is deeply scalloped, especially at the median am- 

 bulacral sutures, where notches soon appear, and the young Encope 

 gradually takes a deeply lobed outline. 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 absence of certain lunules, 

 which are nothing but features of the young either retained in the 

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