310 THE FOSSIL ECHINOIDEA 



Subfamily TBIPLEGHINID^. 



Genus HIPPONOE, Gray, 1840. 



(Syn. Tripneustes, Agassiz, 1841.) 



Test of large size, tumid, and often subconoid. 



Ambulacral areas very broad, nearly equal, or subequal, to the interradial areas. 

 Poriferous zones very wide. Pores arranged in three distinctly spaced vertical series, 

 the external ones regularly vertical, the median series sporadic. 



Primary tubercles imperforate and not crenulate, small and numerous, arranged in 

 horizontal rows in the interradial areas, forming also more or less regular vertical series. 

 Small secondary tubercles distributed between the three vertical rows of pores. 



Peristome small and subcircular ; with deeply indented mouth-slits. 



J. HippoNOE PEOAViA, Duncan & Sladen. Plates XLVIII & XLIX, Fig. 1. 



The test is of large size, and was probably circular in marginal contour, slightly 

 depressed, the height being a little more than two thirds of the diameter ; convex 

 abactinally and probably with a tendency to become conoid towards the apex ; with 

 great tumidity at the ambitus, which passes gradually over on to the actinal surface, 

 where the test is slightly concave. 



Ambulacral areas remarkably broad, their width at the ambitus being greater than 

 that of the interradial areas ; and they are flush with the general surface of the test. 

 The poriferous zones are straight and of great breadth, being a shade broader than the 

 width of the interporiferous area ; thus at the ambitus of the large type specimen the 

 breadth of the interambulacral area measures 49 millim., the whole ambulacral area 

 50'75 millim., and the poriferous zone 17 millim. The pores form three distinct, 

 vertical, and widely spaced series ; the inner and the outer row are almost regularly 

 uniserial, whilst the disposition of the pores in the median series is more or less 

 sporadic, with a tendency to fall into oblique triplets. The manner in which this 

 arrangement is efiiected, and the position of the pairs of pores upon their respective 

 plates will be readily seen on referring to Fig. 4. 



The ambulacral plates are very short, their height being not more than one tenth 

 or one twelfth of the breadth, and each is normally composed of three poriferous plates, 

 each of these contributing a pair of pores to one of the three vertical series above 

 described. Their arrangement is after the following manner : — The inner end of each 

 ambulacral plate, which occupies the interporiferous area, is composed of what we may 

 speak of as the primary poriferous ossicle ; passing the eye along the aboral margin of 

 the plate, and traversing it from the inner extremity of the line outwards, it will be 

 noticed that as soon as the poriferous zone is reached the aboral margin commences to 

 be gradually scooped away, this part of the plate being, along the greater portion of the 

 breadth of the poriferous zone, a mere thin band, less than one third the height of the 

 compound plate. The portion thus scooped away is occupied by two low demi poriferous 

 plates in the form of band-like wedges. At the outer end of the composite ambulacral 



