Dr. T. Wright on the Cidarida- <>f //„< Oolites. I 5t 



interspersed; on the external side of each plate there are nine 

 granulations, which, with those of the adjoining plates, form a 

 sinuous granulated line which defines the internal boundary of 

 the poriferous avenues. The internal row of granulations, with 

 those of the opposite and adjoining plates, form a double granu- 

 lated zigzag space, occupying the centre of the area?, and forming 

 an elevated ridge which serves to separate the two ranges of 

 primary tubercles from each other. 



The mouth is large and decagonal, and is one-half the diameter 

 of the test. The margin is deeply notched with ten indenta- 

 tions. The divisions of the circumference are not equal, as the 

 arch over the ambulacral is one-half greater than the arch over 

 the interambulacral area?. 



The apical disc is greatly developed, occupying more than one- 

 third the diameter of the test ; it is of a pentagonal form, the left 

 anterior angle being more developed than the right. The madre- 

 poriform plate is large, and divided into a posterior porous and 

 an anterior non-porous segment. The posterior pair of ovarial 

 plates are likewise large, the anterior pair are small and imper- 

 fectly developed ; the left plate is rudimentary, in consequence of 

 the anal opening being eccentric and situated before and towards 

 the left side ; the supra-anal plate is in general of a pentagonal 

 form, and composed of from four to six elements united together 

 and set round the posterior border of the anal opening. The 

 ocular plates are triangular and well-developed ; all the plates of 

 the apical disc are studded with small granules. This species 

 belongs to Agassiz's first division of the Salenians which have 

 the sur-anal plate and the oviductal apparatus situated before 

 the madreporiform plate. The primary spines (fig. 1 d) are long, 

 tapering, and slightly compressed, so that a transverse section of 

 one of them forms an ellipsis in the specimen before me. They 

 are in length about twice the diameter of the test. The body of 

 the spine is smooth throughout ; the base is encircled with a pro- 

 minent elevated ring of small oblong closely-set granulations; a 

 smaller circle of larger creuulations surrounding the margin of the 

 concave articulating surface. The secondary spines articulating 

 with the tubercles of the ambulacral area? resemble the primaries 

 in miniature, some of them measuring -j^ths of an inch in length. 

 The dental apparatus is well-developed. The teeth arc strong, 

 triangular, and slightly curved towards the point. 



Affinities and differences. — Acrosalenia hemicidaroides is distin- 

 guished from its congeners by its size, the number and regularity 

 of the primary tubercles, the compound structure of the supra- 

 anal plate, and the granular surface of the apical disc. This 

 Urchin so much resembles a Hemicidaris in the form of the 

 test, the structure of the ambulacra and poriferous avenues, that 

 it was not until we had obtained roechnem with the apical ffiac 



