190 Dr. T. Wright on the Cassidulidte of the Oolites. 



slightly produced ; anal opening pyriform, with the apex di- 

 rected outwards and upwards, excavated out of the base and 

 border of the single area ; base concave ; mouth small, nearly 

 central, situated in a depression. 



Height j^ths of an inch, antero-posterior diameter 1 inch and 

 ~^jths, transverse diameter 1 inch and T \yth. 



Description. — This Urchin resembles the preceding species in 

 its general outline, but a careful inspection shows that it differs 

 in many important particulars from it. The test is sub-hemi- 

 spherical, more or less depressed in different individuals ; the 

 antero-posterior exceeds the transverse diameter, and the apical 

 disc and the vertex are excentrical, being situated nearer the an- 

 terior than the posterior border, thereby occasioning a slight 

 obliquity, which is more or less evident in different individuals ; 

 the sides are tumid, and the border is gently rounded. The 

 base is concave and much depressed in the centre, in which con- 

 cavity the mouth-opening is situated ; it is nearer the anterior 

 than the posterior border, and is less than one-third the trans- 

 verse diameter of the test, its margin being divided into ten 

 nearly equal lobes. The ambuiacral area? taper very gradually ; 

 each pair of the small narrow plates supports a tubercle, which 

 occupies the same relative position thereon on every fourth plate, 

 so that the area? are adorned with six rows of tubercles arranged 

 obliquely in V-shaped lines. The pores form pairs set closely 

 together in single files throughout the avenues ; about the middle 

 of the sides there are from four to five pairs of pores opposite 

 each interambulacral plate. The interambulacral arese are twice 

 and a half the width of the ambuiacral ; in the specimen before 

 me there are twenty-seven plates in each column ; each of the 

 side plates is slightly bent upwards in the middle, whilst those 

 of the base are nearly straight ; each plate carries from four to 

 eight tubercles surrounded by circles of minute granules. The 

 tubercles are arranged on the plates in such a manner that they 

 form curved lines on the arese ; they are more closely crowded 

 together at the basal angle, and are fewer in number, but of 

 larger size, around the mouth, where they are set more widely 

 apart ; each tubercle is perforated and raised upon a mammil- 

 lated eminence with a crenulated summit : even the encircling 

 granules exhibit perforations when viewed with a high magni- 

 fying lens. The spines are unknown. The apical disc is small, 

 and formed of five ovarial and five ocular plates ; the anterior 

 pair of ovarials arc the largest, and the right*, which is the 



* The Urchin is supposed to be placed before the observer, with the an- 

 terior border before, the anal border behind ; the right and left sides of the 

 test consequently correspond to his. 



