Dr. T. Wright on the Cassidulidaj of the Oolites. 211 



Nucleolites dimidiatv*, Phillips. 



Syn. Nucleolites dimidiatus, Phillips, Geol. of Yorksh. vol. i. pi. 3. 

 fig. 16 ; Forbes, Memoir of Geol. Survey, Decade 3. description 

 of pi. 9 ; Agassiz and Desor, Cat. raisonnc des Echinides. 



Nucleolites para pi cuius, Agassiz, Catalogue Syst. p. 4. 



Test ovate or subquadrate, compressed at the sides, rounded an- 

 teriorly, expanded and bilobed posteriorly, dorsal surface con- 

 vex; apical disc central, vertex excentral, anterior and lateral 

 border and posterior lobes tumid; ambulacra narrow and lan- 

 ceolate ; anal valley deep, ovate or obtusely lanceolate, extend- 

 ing about two-thirds of the space between the border and the 

 vertex, and terminating at a distance from the disc ; base con- 

 cave, much depressed at the mouth-opening, which is penta- 

 gonal and excentral. 



Height -njths of an inch, antero-posterior diameter 1 inch and 

 gtjths, transverse diameter 1 inch and y^th. 



Description* — This species was formerly considered a variety 

 of N. clunicularis, but was correctly separated from that form by 

 Mr. Phillips. It does not appear to deviate so much from its 

 typical outline as that species ; the greater fullness of the sides, 

 the tumidity of the posterior lobes, and the shortness of the anal 

 valley are characters which are very uniformly preserved in the 

 individuals now before me. The single and the anterior pair of 

 ambulacra are narrower than the posterior pair, they have a lan- 

 ceolate form, rather obtuse at the apex ; the pores are set at short 

 distances apart ; on the dorsal surface they become approximated 

 near the margin, and pass from thence to the mouth in pairs, set 

 however more widely apart at the base. The interambulacra are 

 of unequal width, the posterior pair are nearly one-third wider 

 than the anterior pair ; the single area has a cordate form from 

 the development of the posterior lobes, and equals in width that of 

 the posterior arese. The anal valley forms one of the distinctive 

 characters of this species ; it is of an ovate or lanceolate form 

 with a blunt apex, and in some individuals it has the appearance 

 of a portion of the test having been drilled out of the single area ; 

 in some specimens it extends only half the distance between the 

 margin and the vertex, whilst in others it reaches two-thirds 

 that length, but in all the individuals before me there is an un- 

 depressed portion of the test separating the apical disc from the 

 superior border of the anus ; inferiorly the valley forms a consi- 

 derable sulcus, which grooves the centre of the single area, divi- 

 ding its posterior border, and producing the cordate bilobed 

 form it assumes ; the surface of the test is irregularly covered 

 with small tubercles, which are of microscopic minuteness above, 



R 



