298 Dr. T. Wright on the Cassidulida? of the Oolite*. 



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

 T 3 ^ths, transverse diameter 1 inch and ^ths (Inferior Oolite spe- 

 cimen) . 



Height |§ths, antero-posterior diameter 1 inch and y^ths, 

 transverse diameter 1 inch and y^th (pyramidal variety from the 

 Cornbrash). 



Description. — This Urchin has been long known to naturalists, 

 and the list of synonyms shows some of the names under which 

 it has been described; it presents many varieties of outline as 

 well as of height, and these forms have sometimes been mistaken 

 for and described as distinct species by different authors, so that 

 much confusion has been made regarding its identity. The suite 

 of specimens before me, from the Inferior Oolite, Great Oolite, 

 and Cornbrash, vary from a suborbicular to a nearly quadrate 

 outline, presenting all the intermediate forms : they are rounded 

 anteriorly, a little contracted before, slightly swelled out at the 

 sides, and more or less bilobed posteriorly. The dorsal surface 

 is convex, exhibiting various degrees of elevation ; in some it is 

 much depressed, in others elevated into a subcorneal form. In the 

 ten specimens before me, there are not two that have the same 

 proportional height. The vertex is almost always excentrical, and 

 inclined towards the anterior border, but the amount of this in- 

 clination, like the height, varies in different individuals : in a 

 beautifully perfect specimen from the Cornbrash the vertex is 

 quite central. The ambulacral area? have a petaloid or lanceolate 

 form with nearly parallel sides, the single area and the anterior 

 pair are nearly of the same length and width, and the posterior 

 pair are the longest and widest. The pores on the upper two- 

 thirds of the dorsal surface are situated at some distance apart, 

 and are united by fine sutures ; at the inferior third of the area 

 they approximate and pass from thence round the basal angle to 

 the mouth ; on the under surface the pores are very small and 

 indistinct and set widely apart. The interambulacral arese are 

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

 than the anterior pair, and the single area is the widest. The 

 anal valley extends from the disc to the basal angle ; it is of a 

 lanceolate form with vertical parallel walls, which gradually di- 

 verge and then approximate, and afterwards expand outwards, 

 forming a well-defined groove ; the round anal opening is per- 

 forated at the extreme end of the valley ; the whole surface 

 of the test is covered with small close-set tubercles, surrounded 

 by circular depressions, and having the interspaces minutely 

 granulated ; those on the dorsal surface are quite microscopic, 

 whilst those on the base are larger and better defined. The 

 apical disc is formed of two pairs of perforated ovarial plates, and 

 a single imperforate plate with a central element having a spongy 



