Dr. T. Wright on the Cassidulidse of the Oolites. 195 



Hybociypus gibherulus y Agass. 



Syn. Hybociypus gibberulus, Agassiz, Echinoderm. Foss. dela Suisse, 

 Part 1. p. 75. pi. 13. fig. 10-12 ; Desor, Monograph des Gale- 

 rites, p. 84. pi. 13. fig. 12-14. 



Test nearly orbicular, elevated and contracted anteriorly, en- 

 larged, depressed, produced and truncated posteriorly; the 

 single ambulacral area the highest, and forming a gibbous 

 crest by an elevation of the anterior pair of interambulacral 

 area?; anal valley wide and deep; single interambulacrum 

 slightly produced, deflected and truncated ; base much un- 

 dulated ; mouth and vertex excentrical. 



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

 ^£ths, transverse diameter 1 inch and ^$ths. In consequence of 

 the fracture of the summit of the crest, the true height cannot be 

 exactly ascertained. 



Description. — This species is remarkable for the prominent 

 gibbous crest formed by the anterior interambulacral and single 

 ambulacral areae, and which gives value to its specific name ; on 

 the anterior border of the crest a groove is formed which extends 

 from thence to the mouth ; the anterior lateral are more con- 

 tracted than the posterior lateral borders, and the posterior half 

 of the test is less elevated than the anterior half, and gradually 

 declines from the vertex to the single interambulacral area, which 

 is abruptly truncated. The single and the anterior pair of am- 

 bulacra form nearly straight lines from the vertex to the circum- 

 ference ; the posterior pair are gently sinuous ; the pores are dis- 

 posed in close-set pairs on the dorsal surface, but are more wide 

 apart at the base. The interambulacral arese are of unequal 

 width ; the anterior pair are the narrowest, the posterior pair are 

 one-third wider than the anterior, and the posterior single area 

 is the widest ; its dorsal part is occupied by the anal valley, which 

 is wide and deep above and expanded below, and forms an in- 

 considerable prominence which is abruptly truncated and much 

 deflected posteriorly ; the anus is a large, oval opening, perfo- 

 rated at the extremity of the valley. The mouth is situated im- 

 mediately under the vertex, and both are slightly excentrical ; 

 the mouth-opening has a pentagonal form, and lies in a consi- 

 derable concavity, the base being much undulated from the con- 

 vexity of the interambulacra and the straightness of the valleys 

 formed by the ambulacra. In the specimen before me the apical 

 disc is unfortunately broken with the summit of the gibbous 

 crest. The test is very thin, and covered with small homogeneous 

 tubercles, which are larger, more numerous, but less regular on 

 the base than on the dorsal surface ; they are all surrounded by 

 a very apparent circular depression. 



Q 



