312 Dr. T. Wright on the Cassidulidse of the Oolites. 



contracted at the vertex and towards the border ; apical disc small, 

 subcentral, and formed of four perforated and a single imper- 

 forated ovarial plate, with a small madreporiform body occupy- 

 ing the centre of the disc, and five ocular plates the angles 

 between the ovarials at the circumference thereof. Interambu- 

 lacra wide, the single area produced and deflected ; anal opening 

 round or longitudinally oval, situated in the infra-marginal 

 border of the rostrated process. Ventral surface concave or sub- 

 concave ; mouth subcentral and pentagonal, surrounded by five 

 prominent lobes and a rosette of well-developed pores on the am- 

 bulacra near the opening. Tubercles close-set and microscopic. 

 Most of the species are Oolitic, a few are found in the Neocomian 

 strata, but all are extinct. 



Pygurus Blumenbachii, Agassiz. PI. IV. fig. 2 a-c. 



Syn. Clypeaster Blumenbachii, Koch and Dunker, Norddeutschen 



Oolithgebild. pi. 4. fig. 1. p. 37. 

 Pygurus Blumenbachii, Agassiz and Desor, Cat. raisonne, A. N. S. 



torn. viii. p. 162. 



Test thin, nearly ox'bicular, with a sinuous margin ; dorsal surface 

 elevated anteriorly, gradually declining posteriorly, border 

 sinuous, with the centre produced and deflected; base con- 

 cave, much undulated ; mouth-opening excentrical, subpen- 

 tagonal, with a five-lobed margin ; anal opening submarginal, 

 round or nearly ovate. 



Height 1 inch and T \jth, antero-posterior diameter 2 inches, 

 transverse diameter at the widest part 2 inches and joth. 



Description. — This Urchin has a very remarkable form ; the 

 anterior border is slightly excavated in the centre, with two con- 

 vex lobes on each side thereof. The lateral parts of the test are 

 convex, and the posterior borders form two sinuous lines having 

 the centre of the single interambulacrum produced posteriorly 

 into a beak-shaped, slightly deflected process. The ambulacra 

 have an elegant petaloid form with approximated apices ; the pori- 

 ferous avenues are marked by well-developed transverse sulci. The 

 ambulacral arese, which are wide above, rapidly converge towards 

 the lower third ; the pores here approximate and pass from thence 

 in close-set pairs to the border of the test. Along the base the 

 pairs are placed wide apart, but again become more closely ap- 

 proximated as they approach the mouth. 



The interambulacra are unequal ; the anterior pair are con- 

 vex and prominent ; they rise nearly perpendicular, forming an 

 angle of 80°, and near the vertex curve backwards ; the pos- 

 terior pair and the single area slope at an angle of 35°. The 

 dorsal surface thus acquires the anterior elevation which gives so 

 remarkable a character to this species, and connects it with P, 



