Dr. T. Wright on the Cassidulidse of the Oolites. 95 



-j-^ths, transverse diameter 1 inch and x\jths (Inferior OoHte spe- 

 cimens). Height y%ths of an inch, antero-posterior diameter 

 1 inch and y%ths, transverse diameter 1 inch and y'^ths (Corn- 

 brash specimens). 



Desciiption. — The inconsiderable prominence of the ambulacra 

 of this Urchin gives the circumference a subcircular outline ; the 

 arese are furnished with from six to eight rows of tubercles, four 

 of which only extend from the mouth to the vertex ; the other 

 rows disappear at various distances from the circumference, 

 where they are most fully developed. The interambulacra are 

 twice and a half the width of the ambulacra, and furnished at 

 the circumference with sixteen rows of tubercles, a few of which 

 only form distinct horizontal ranges on the dorsal surface of the 

 test ; the perforated tubercles are encircled with fine granulations 

 which cover likewise the intertubercular surface of the plates ; 

 the base is concave, and the tubercles of both arese attain their 

 greatest development in this region, presenting in their size a 

 remarkable contrast to the microscopic character of those occu- 

 pying the upper surface of the test. 



The apical disc is central, and occupies the vertex ; it is com- 

 posed of four perforated ovarial plates, and a single imperforate 

 plate with five ocular plates ; the madreporiform body is largely 

 developed and projects from the single imperforate ovarial to- 

 wards the centre of the disc, where the line of suture between the 

 other plates is not clearly defined ; the madreporiform body has 

 the appearance of a central spongy mass, round which the ovarial 

 and ocular plates are disposed, and has been figured and de- 

 scribed as such by Agassiz and Desor ; but specimens before me, 

 from the Inferior Oolite of Dundry and the Cornbrash of Wilts, 

 prove this to be an error. The single ovarial' plate, with its 

 madreporiform body, is of an irregular pentagonal form, the 

 spongy portion projecting inwards to the centre ; the anterior and 

 posterior pair of ovarial plates are of a pentagonal form, and are 

 perforated at their apices for the passage of ducts ; the ocular 

 plates are likewise pentagonal and wedged between the ovarial 

 plates at the summits of the ambulacra, where the eyeholes are 

 situated. The mouth occupies the centre of the concave base, 

 but is rather nearer the anterior than the posterior border ; it is 

 about one-third the diameter of the test, and its margin is di- 

 vided into ten lobes. The anal opening is large, occupying 

 nearly all the basal portion of the single interambulacrum ; it is of 

 a pyriform shape, having the apex directed towards the mouth, 

 and the round base towards the border of the test. The pori- 

 ferous avenues are occupied by a row of pores placed in pairs, 

 and extending without interruption from the margin of the 

 mouth to the apical disc. The spines are unknown. 



Affinities and differences, — This species resembles H. hemi- 



