216 Dr.T. Wright on the Cassidalidse of the Oolites. 



Nucleolites Solodurinns, Wright. 



Syn. Clypeus Solodurinus, Agassiz, Echin. Foss. Suisse, vol. i. p. 35. 

 tab. 5. fig. 1-3. 



Test oblong, posterior border much produced, deflected and trun- 

 cated; ambulacra petaloid, apices approximated; vertex and 

 apical disc excentral, inclined towards the posterior border; 

 anal valley narrow, acutely lanceolate, extending from the ver- 

 tex to the posterior border ; base concave ; mouth excentral, 

 surrounded by five well-developed lobes. 



Height ^gths of an inch, an tero -posterior diameter 2 inches, 

 transverse diameter 1 inch and T 8 <yths. 



Description. — This Urchin is distinguished from its congeners 

 by its oval circumference, depressed dorsal surface, and produced 

 and truncated posterior border. The ambulacral areas are formed 

 of very narrow plates, have a regular acutely lanceolate or pe- 

 taloidal form, a nearly uniform width and closely approximated 

 apices; and the pores are placed further apart, and connected 

 by finer and more close-set sulci than in N. Hugii, which gives- 

 more breadth to these divisions of the test ; above the marginal 

 angle the pores approximate, on the base they are wide apart, 

 but within a short distance, about 4 lines, from the mouth 

 they become more numerous and form close-set triple oblique 

 pairs. The interambulacra are of unequal width, the anterior 

 pair are the narrowest, the posterior the widest, and the single 

 interambulacrum about the same as the latter ; this area is much 

 produced, deflected, and abruptly truncated posteriorly ; the anal 

 valley is narrow, deep, and acutely lanceolate, extending from the 

 apical disc to the border. The vertex is excentral and slightly 

 inclined posteriorly ; the apical disc is small and situated imme- 

 diately behind the vertex ; it is formed of an anterior pair and a 

 posterior pair of oblong perforated ovarial plates, and a single 

 imperforate ovarial plate, with a spongy madreporiform body oc- 

 cupying the centre of the disc. The ocular plates are small, and 

 the eyeholes near the margins. The base is concave and undulated, 

 the ambulacra forming straight valleys, and the interambulacra 

 convex elevations between. The mouth is pentagonal and ex- 

 central; its margin is surrounded by five lobes, the terminal por- 

 tions of the interambulacra, between which are notches formed 

 by the contracted ambulacra, which are here freely perforated for 

 the passage of tubular organs, which appear to have been very nu- 

 merous around the mouth. The test is moderately thick ; its 

 surface is covered with small tubercles surrounded by circular 

 depressions, as in the other species of this genus ; the tubercles 

 on the base are larger than those on the dorsal surface. 



Affinities and differences. — N. Solodurinus more nearly resem- 



