208 Dr. T. Wright on the Cassidulidaj of the Oolites. 



cular, subpentagonal or oblong, with almost every intermediate 

 form blending the two extremes in different individuals; the dorsal 

 surface is uniformly smooth and convex, elevated posteriorly, 

 gently declined anteriorly, and more or less depressed, the vertex 

 being situated in general nearer the posterior than the anterior 

 border; the sides are tumid, the anterior part is flattened, and 

 the posterior is produced ; the antero-lateral region is narrower 

 than the postero-lateral ; the prominence of the interambulacra 

 in some individuals produces the subpentagonal varieties. The 

 ventral surface is very unequal, the convexity of the interambu- 

 lacra amounting to nodulose eminences ; the posterior single area 

 in particular is very gibbous, prominent, and much deflected ; its 

 posterior surface is truncated and channelled to form the anal 

 valley, which is bounded by two ridges, commencing at the 

 apices of the posterior ambulacra, and passing downwards and 

 outwards towards the base, where they may be traced on the 

 summit of the gibbous area as far as the mouth ; in the upper 

 part of the marginal valley and nearly on a level with the dorsal 

 surface the anal opening is situated, having a pyrifoi'm shape, 

 with the apex directed upwards. The ambulacra are of unequal 

 width, the posterior pair being the widest ; the anterior arese are 

 about the same width ; the single ambulacrum is gently sinuous, 

 the anterolaterals curve gracefully towards each other, and the 

 three arese converge at the apical disc near the centre of the 

 dorsal surface ; the posterior pair are somewhat wider than the 

 others, they curve gracefully round the single interambulacrum, 

 form an arch over its produced and truncated border, and con- 

 verge above the anal opening; the ambulacra are all complete, 

 and pass continuously from the mouth to the two dorsal sum- 

 mits ; they are formed of pairs of small plates, about one-fourth 

 the depth of the interambulacral plates, each plate being perfo- 

 rated at its outer side with two small pores ; the ambulacra be- 

 come wider about the mouth, and the pores increase in number, 

 forming three oblique series of three pairs in each. The inter- 

 ambulacra are of unequal width ; on the dorsal surface they are 

 on a level with the ambulacra, but on the ventral surface they 

 form nodulose eminences between them — so much so, that the 

 ventral is as remarkable for its undulations as the dorsal is for 

 the smoothness of its surface ; the single interambulacrum being 

 prominent and gibbous superiorly, posteriorly and inferiorly, but 

 especially so in the latter region. The apical disc is a very cu- 

 rious structure, and is formed of two pairs of perforated ovarial 

 plates, disposed in pairs, at some distance apart, and separated 

 by three largely developed ocular plates which extend into the 

 centre of the disc ; the anterior pair of ovarials are of an irre- 

 gular form, and separate the single ambulacrum from the antero- 



