538 MELLITA TESTDDINATA. 



Long. Trans. Di*t. Apex Post. Lunule Anns 



Diaui. Ihiin Vost KJge. from Edge. fromEdge. 



50. 51. 28. 2. 11.2 22.5 'J. 12.2 



Guayaquil; Panama. 



Mellita testudinata 



! Mellita testudinata Klein, 1734, Nat. Pisp. Eeh. 

 (See Part II. p. 322.) 



PL XI. f. is -22; PL XI P. ; PL XIP.f. 1-2; PL XXXVII. f. 1, 2. 



Brazil ; West India Islands ; North ami South Carolina. 



(MELLITA.) Astriclypeus. 



Astriclypeus VittitlLL, 18C7. Notes on Rad., p. 311. 



This genus was firsi established by Verrill, and shortly afterwards by 

 Troschel, for a sea-urchin, combining at first glance the features of several 

 of the Scutellidae, hut which I am inclined, after an examination of the in- 

 ternal structure, to associate with Mellita as a subgenus. It forms the passage 

 between the Scutellidae of the type of Mellita and of tin- type of Echino- 

 disciis. It h;is, its in Mellita hexapora, five ambulacral lunules ; hut no pos- 

 terior interambulacral lunule, which is found in till the species of Mellita 

 thus far known. It litis the same general system of marginal limestone sup- 

 ports existing in Mellita. hut has not. like Mellita, the isolated pillars sepa- 

 rating the jaws from the rest of the chamber containing the alimentary canal. 

 The whole lower floor is smooth or nearly so. as in some species of Echino- 

 discus. without pillars, between the inner edge of the marginal limestone net- 

 work and the auricles. The auricles are low, completely disconnected, with 

 a notch in the centre, much as we find it in Mellita Stokesii. The jaws are iden- 

 tical in structure with those of Echinodiscns ; their flatness is extreme; they 

 have, as in that genus, the peculiar pits, articulating them upon the auricles, 

 which are covered by the jaws, and do not hold them in place as in Mellita 

 proper. The teeth themselves tire short, flat This genus is specially im- 

 portant, as connecting most closely the Encopidae with the Scutellidae of 

 the type of Mellita and of Echinodiscns. which seemed, up to the time of 

 the discovery of this genus, widely separated. 



