THE AMBULACRA 0¥ FOSSIL ECHINOIDEA. 435 



Thus the plate a' is a demi, plate 6 is a large primary, and 

 plate c is a low and broad primary with an arched aboral edge. 



Compound plate II. — This is composed of three primaries, of 

 which the middle is the largest and occupies most of the plate near 

 the median line. The plate is triily Diadematid in its shape and in 

 the details of the sutures. Plate a is a low broad primary with its 

 adoral edge curved actinaUy. Plate c is also a low and broad 

 primary and the aboral edge is curved with the convexity placed 

 towards the apex. Plate b is low at the poriferous area, nipped in 

 and lower further towards the centre of the plate, and expanding 

 considerably towards the median line. 



Compound plate III. — This resembles the last ; but the direction 

 of the suture uniting the middle plate with the adoral plate is less 

 curved and approaches a straight line. 



Compound plate IV. — This is Arbacioid in shape and there is an 

 aboral demi-plate (a'), a large middle primary plate (6), and an 

 adoral demi-plate (c'). 



The middle plate, much the largest, carries the tubercle, and the 

 whole of the vertical suture is in relation to it. The poriferous part 

 of this hatchet-shaped plate (6) is low. Plate a' has its actinal edge 

 much curved adorally, and the suture nearly reaches the median 

 line, but the plate is a demi-plate. 



Plate c', also a demi-plate, has its aboral suture curved with the 

 convexity aboral, and it terminates short of the median line and at 

 about the same vertical position as the suture of plate a'. 



It is evident that Pcdina is a well-defined genus, and that the 

 situation of the sutures which are in the ambulacra is diff'erent 

 from that seen in the family Echinidse, and in the main resembles 

 that of Diadema and Pseudodiadema. 



The presence of the demi-plates fashioned after the Arbacioid 

 type ally the form with Hemicidaris. 



Genus Stomechhsttis, Desor. 



The examination of the ambulacra of the typical species of this 

 Oolitic genus shows that the plates are not arranged after the method 

 which characterizes the true Echinidse, of which the common 

 Echinus is the type. The compound plates of Stomechinus are made 

 up of primary plates combined and modelled after the Diadema type 

 and not after that of Echinus or Strongylocentrotus. 



Desor placed this Oolitic genus with Echinus and Psammechinus, 

 and apart from the Diadematidae. 



The following is his diagnosis (' Synopsis des Echinides,' p. 124) : — 



Urchins of moderate size, subcorneal, with pores distinctly tri- 

 geminate as in the true Echini. Peristome large, profoundly cut, 

 no longer decagonal, but in the form of a pentagon, the bifid angles 

 of which correspond to the interambulacra. Spines striated longi- 

 tudinally and small. 



Stomechinus higranularis, Lamk., sp., is the type, audits synonyms 

 are Echinus serialis, Wright, E. intermedius, Agass., and E. arenatus, 



