88 Dr. T. Wright on the Cassidulidaj of the Oolites. 



^»^ft rrf h ttrtr Qfi fl^g CaSSIDULID^E. 



The Cassidulidse have been recently dismembered by Agassiz 

 and Desor from the family Clypeastroidse, in consequence of the 

 mouth of the Cassidulidse being destitute of the jaws and teeth 

 possessed by the true Clypeastroidse* : how far this supposed ne- 

 gative edentulous character is of sufficient zoological importance 

 to justify the separation of the Cassidulidse from the great natural 

 family which they resemble in most of the important points of 

 their structure, it is not our intention on the present occasion to 

 inquire ; it has however the palseontological merit of grouping 

 together many genera of Echinida closely related to each other 

 by structural affinities as well as by their limited distribution in 

 time. 



Family Cassidulid^. 



Urchins having an oblong, subpentagonal or orbicular cir- 

 cumference, generally clypeiform, with a uniform convex dorsal 

 surface, mostly depressed, but sometimes elevated or conoidal ; 

 the shell is of moderate thickness, and its surface is covered with 

 small tubercles and granules. The tubercles are smaller on the 

 dorsal than on the ventral surface, and are surrounded by a cir- 

 cular depression ; sometimes the tubercles are arranged in ver- 

 tical ranges, but in general they are sporadical. The granules 

 are entirely microscopic, and occupy the intertubercular spaces ; 

 to the whole of the tubercles small filiform spines are attached. 

 The ambulacral arese are narrow, and the pores are disposed in 

 close-set pairs, forming a single file as in the Galerites ; or they 

 are placed at some distance apart, and united by transverse 

 sutures, which occasions the petaloid forms seen on the dorsal 

 surface of the test of the Nucleolites. 



The mouth is central or subcentral, and is sometimes armed 

 with jaws and teeth ; in some genera its margin is divided into 

 ten lobes ; in others it is edentulous and the opening is round or 

 pentagonal, and sometimes surrounded by five prominent lobes, 

 formed by the folding inwards of the terminal portions of the 

 interambulacra. The anal opening is large, and situated on the 

 dorsal or ventral surface ; it is rarely marginal, often infra-mar- 

 ginal, and when dorsal is lodged in a valley. The apical disc is 

 formed of four perforated ovarial plates and a single imperforate 



* This generalization is not strictly in accordance with the facts. The exist- 

 ence of a dental apparatus was discovered by Mr. Chas. Stokes in Galerites 

 albogalerus, and specimens of this cretaceous Urchin, with the lantern and 

 teeth, are in the cabinets of Messrs. Stokes and Bowerbank. It is probable 

 that all the members of the Galerite group with a central decagonal mouth- 

 opening possessed a similar dental armature. 



