THE AMBTJLACEA OF FOSSIL ECHTNOIDEA. 



43S 



It appears consistent with the results of these researches to decide 

 that the genus Pseudodiadema should be restricted to the forms with 

 triple plates, with an occasional extra plate in the nature of a demi- 

 plate ; that is, to groups one and two. Of such a genus the spe- 

 cies placed in the following list may be taken as common forms : — 

 Pseudodiadema Moorei, Wright ; P. depressum. Ag. ; P. radiatum, 

 Wright ; P. Balcerioe, Woodw. ; P. priscum, Cott. ; P. incequale, Des. ; 

 P. WrigJitii, Cott. ; P. prisciniacense, Cott. ; P. rotulare, Ag. ; P. 

 Benettia;, Porbes ; P. ornatum, Goldf. ; and the species which has 

 been partly described in this communication. Pseudodiadema 

 Brongniarti, Ag., and P. Bailyi, Wright, may be taken as good 

 examples of the subgroup with an occasional demi-plate ; and of 

 course P. Jiemisplicericum is the type. 



On the other hand, the species with at least four and with five 

 pairs of pores or more, both in forms not full-grown and adult, 

 should be ranged in another genus — Plesiodiadema. Under this 

 genus will be arranged the species P. mamillanum, Eomer ; P. 

 Michelini, Ag. ; P. Blmicheti, Des. ; P. Verneuilii, Cott. ; P. tenuis, 

 Des. : P. annulare^ Desor, &c. 



Pig. 9 (see p. 452). 



III. 



- IV. 



Genus Pedina, Agass. 



The diagnosis of Pedina (an Oolitic genus) has been partly 

 noticed (page 422), and it is now necessary to determine what dis- 

 tinction the obliquity of the triple pairs of pores makes in the shape 

 of the plates. 



Taking Pedina Smithii, Wright, as the type, an examination of 

 the specimen in the late Dr. Wright's collection indicates that 

 the ambulacra are different from those of Hemii^edina. The 

 arrangement of the triplets of pores is to a certain extent like that 

 in typical recent Triplechinidae, and the first or aboral pair (fig. 9, c) 

 of each series of three in one line is, as in the recent forms, the 

 adoral or actinal pair of a triple compound plate, and is situated 

 much nearer the median line of the ambulacrum than the pair 

 immediately above (fig. 9, h). The pairs are in oblique series of three, 



