Dr. T. Wright on the Cidaridse of the Oolites. 245 



we may observe in young Urchins small plates loosely connected 

 together and supporting incomplete spines. 



The numerous genera of the family Cidaridse are distributed 

 by M. Agassiz into four groups : — 



1. The Cidarid^e are characterized by their thick test, nar- 

 row ambulacra, and large principal tubercles in the interambu- 

 lacral arese. 



2. The Saxenians are characterized by the development of 

 their apical disc, and the presence of an additional central or sur- 

 anal plate in the same. 



3. The Echinid^: have a thin test, and numerous small 

 principal tubercles in the ambulacral and interambulacral arese. 



4. The Echinometrans have an elongated oblong form in a 

 direction oblique to the antero-posterior diameter of the test. 



Family Cidarid^e*. 



Form circular. Mouth central, situated at the inferior pole, 

 closed by a buccal membrane which is either naked or covered 

 with granules. Anus opposite the mouth, opening in a ring 

 composed of ten plates, five of which appertain to the genital, 

 and five to the visual organs. The antero-posterior diameter is 

 indicated by the median madreporiform body which becomes 

 united to the single ovarial plate. The plates of the test sup- 

 port tubercles disposed in regular order for carrying moveable 

 spines of various forms, some of which are proportionably large. 

 The organs of mastication consist of five jaws, each armed with 

 a long tooth. This framework is articulated to the test by several 

 arched processes called auricles. 



Genus Cidaris, Lamk. 

 Form circular, test thick, flattened at both poles. Ambulacral 

 arese narrow, about one-fourth the diameter of the interambulacral 

 arese, and covered with small close-set granules. Pores disposed 

 in simple pairs. The principal tubercles in the interambulacral 

 columns are perforated, and carry large heavy spines which are 

 smooth or furrowed, spiny or granular. The ovarial plates are 

 large, pentagonal and equal ; the ocular plates are small and tri- 

 angular, and wedged between the ovarial. The mouth is cir- 

 cular and without indentations ; the buccal membrane is covered 

 with imbricated scales upon which the ambulacral pores extend. 

 Jaws powerful, composed of five pyramids, the branches of 



* The group of Cidaridsc includes six genera : Cidaris, Lam., Gonioci- 

 daris, Desor, Hemicidaris, Agass., Acrocidaris, Agass., Acropeltis, Agass., 

 Palteocidaris, Agass. 



