BRITISH FOSSILS. 



Decade III. Plate IX. 



DYSASTER RINGENS. 



[Genus DYSASTER. Agassiz. (Sub-kingdom Radiata. Class Ecliinodermata, Order 

 Echinidse. Family Cassidulidse.) Body sub-discoidal ; ambulacra simple, continuous, 

 radiant; the posterior lateral pair separated from the rest, and converging to form a sum- 

 mit distinct from that formed at the genital disk by the two anterolateral and the 

 anterior one; mouth sub-central, inferior; anus at the hinder extremity ; tubercles perfo- 

 rate.] 



Synonyms. Dysaster ringens, Agassiz, Ech. Foss. de la Suisse, 1st part, 

 p. 5, t. 1, figs. 7-11 (1839) ; Cat. Syst., p. 3 ; Desor, Monog. des Dysaster, 

 p. 24, pi. 1, figs. 13-17 ; Cotteau, Etudes sur les Echin. Foss., p. 46, pi. 2, 

 figs. 10-13 ; Agassiz and Desor, Cat. Kais. des Echin ; Ann. des Sc. Nat., 

 3rd series, vol. viii., p. 33. Collyrites ringens, Desmoulins, 3rd Mem. 

 sur les Echin., p. 368. Dysaster Eudesii, Agassiz, Cat. Syst., p. 3; 

 Desor, Monog. des Dysaster, p. 23, pi. 1, figs. 5-12. Dysaster subringens^ 

 M'CoY, Annals Nat. Hist., 2nd series, vol. ii., p. 415 (1848). 



Diagnosis. D. testa depressd orbiculari, seu subpentagonali, lateribus 

 tumidis, dorso convexiusculo, vertice plerumque centrali ; ambulacris pos- 

 terO'lateralibus supra anum convergentibus ; ano i?i sulco terminali ; ventro 

 concavo, areis inter ambulacralibus tumidis, area postica gibbd, ore subcen- 

 trali. 



The curious genus to which the fossil before us belongs, although of 

 very late constitution, having been first founded by Agassiz in 1834, 

 has scarcely as yet settled down into a decided position among sea- 

 urchins. The species most anciently known were styled Spatangites by 

 Leske, assigned to Ananchytes by Lamarck, and Nucleolites by Munster. 

 Since Dysaster was constituted for their reception, it was first 

 placed among the " Spatangoides" by Agassiz, referred to the " Cly- 

 peastroides" by Desor, and back again to the former family by Agassiz 

 and Desor jointly. Very recently, M. Gustave Cotteau, in his ex- 

 cellent work on the fossil Echinidce of the department de L'Yonne, has 

 inclined to regard them as members of the Cassidulidae. After an 

 earnest consideration of their structural relations, I do not hesitate to 

 place them in the last-named family, in which they have distinct and 

 close aflSnity with Hyboclypus and Galerites. The form of the ambu- 

 lacra, combined with the structure of the mouth, is sufficient to warrant 

 such a conclusion. In the foreign Dysaster semiglobus {Nucleolites 



[ill. ix.] s 



