430 PEOP. p. M. DTJIS'CIX ON THE STErCTTJEE OF 



uncoalesced primaries. Near the peristome the simplicity of the 

 compound plate persists, and there is no crowding of the pores. An 



Fig. 7 (see p. 452). 



interesting structure exists in the form of a tag, which, as in the 

 recent Diademata, passes up from the branchial cut by the side of 

 the ambulacra. So far as the test is concerned, or rather that 

 part which remains, the apical system being deficient, there is 

 no distinction to be made between this species and a recent 

 Diadema. 



There are many species included in the genus Pseudodiadema 

 by authors which have the simple triplet arrangement of pores 

 just noticed, and the peristomial crowding never amounts to a dis- 

 placement of pairs or the production of demi-plates. I have been 

 able to examine many of the forms described by Dr. Wright, thanks 

 to the courtesy of his executors. The type of this group existed 

 from the Inferior Oolite, if not from the Lias, to the Cretaceous 

 age inclusive ; and it is a matter of great interest to have been 

 told that hollow, striated, and verticillate spines were found in the 

 Chalk and drawn by Mr. Bone. The Pseudodiademata with simple 

 triplets form, therefore, one distinct type or group. The Pseudodia- 

 demata^ having also occasionally an additional pair of pores belonging 

 to a demi-plate, belong to a closely allied section or subgenus or group. 



The next tj'pe to be considered, the third group, is one in which 

 there are never less than four plates and four pairs of pores to a 

 compound plate, and of which Pseudodiadema mamillanum already 

 alluded to, is an example. 



In most of the specimens of this series there is barely any trace 

 of the divisional sutures to be seen in those plates which have the 

 greatest number of pores ; but an instance of a form clearly pre- 

 senting all the necessary structures to view from which a drawing 

 can be made is in Dr. Wright's collection*. Having examined the 

 specimen, it is evident that a compound plate at the ambitus has no 

 less than five primary plates entering into its composition, and that 

 the next above or aboral compound plate has four (fig. 8). The pairs 

 of pores are in slight arcs, the peripodia are well developed and often 

 occupy nearly or quite the whole height of the poriferous area of 

 the plate on which they are placed. The adoral pore of a pair is 

 always in relation with the suture between its plate and that 



* Pseudodiadema Michelini, Agass. 



