FROM THE INFERIOR OOLITE. 299 



is less elevated than the anterior half (fig. 2 e), and gradually declines from the vertex to 

 the posterior border, which is a little produced and truncated (fig. 2 a, 6). 



The ambulacral areas are of unequal width ; the single area is the narrowest, the 

 anterior pair are a little wider, and the posterior pair are the widest ; the single area 

 makes a straight line from the mouth to the vertex ; its upper half forms the ridge 

 of the anterior gibbous crest (fig. 2 d) ; where it turns round the border a depression 

 is formed, both there and at the base (fig. 2 b, d), by the bulging out of the inter- 

 ambulacra ; the anterior pair, between the border and the disc, are gently curved back- 

 wards (fig. 2 e), and the posterior pair are sinuous (fig. 2 a) ; the single and anterior pair 

 terminate around the front of the disc (fig. 2 a), which is elongated in the longitudinal 

 direction ; the posterior pair terminate in the large specimen (fig. 2 a) within the longi- 

 tudinal valley, one quarter of an inch behind the anterior pair ; in each area there are six 

 rows of tubercles, arranged so that they form oblique v-shaped lines (fig. 2/). 



The poriferous zones are narrow, and, from the border to the disc, the pores are 

 placed close together, whilst, from the border to the mouth, they are wide apart (fig. 2 a) ; 

 this arises from the ambulacral plates on the dorsal surface being narrow, whilst those at 

 the base are broad ; there are six pairs of pores opposite each of the large fourteen plates on 

 the upper surface, which makes eighty-four pairs of pores on the dorsal portion of the 

 zones, whilst there are only about fifteen pairs of pores in the base. 



The inter-ambulacral areas are of unequal width ; the anterior pair are the narrowest ; 

 the posterior pair are one third wider than the anterior, and the single inter-ambulacrum 

 is the widest (fig. 2 a) ; the middle of its dorsal portion is occupied by the longitudinal 

 valley, which is wide and deep above, and shallow and expanded below; the small and 

 numerous tubercles are arranged very closely and regularly together in four or five 

 concentric rows (fig. 2 /) ; and I have counted upwards of one hundred tubercles on 

 one large plate near the margin. 



The tubercles are very numerous at the border and external third of the base, whilst 

 they are fewer, larger, and less regular between that point and the mouth ; they are 

 all perforated, and raised on low, crenulated bosses, surrounded by sunken circular 

 areolas (fig. 2 g) ; the inter-tubercular surface of the plates is likewise covered with 

 close-set miliary granules (fig. 2y). 



The base is concave, and much undulated (fig. 2 b, c), the basal portions of the inter- 

 ambulacra being very convex (fig. 2 d), whilst the ambulacra lie in narrow valleys between 

 them ; the small mouth opening is sub-central, and situated nearer the anterior border, 

 almost opposite to the vertex (fig. 2 b) ; the peristome has a sub-pentagonal form, but its 

 minute structure is more or less concealed in all my specimens. 



The apical disc is nearly central ; it lies behind the summit of the crest, which rises 

 above it (fig. 2 e), so that the vertex is before the disc in this remarkable species ; it has 

 a narrow, elongated form, and is composed of six ovarial and five ocular plates (fig. 2 e 



39 



