BRITISH 



FOSSILS. 



Decade TV. Plate X. 



■ CARDIASTER EXCENTRICUS. 



[Genus CAEDIASTER. Eorbes (1850). (Sub-kingdom Radiata. Class Echinoder- 

 mata. Order Echinoidea. Eamily Ananchytidse.) Body cordate, tumid, or depressed; 

 lateral ambulacra having the upper part of the avenues slightly dissimilar ; all the 

 ambulacra convergent on the vertex, the anterior one lodged in a strongly marked sulcus 

 with angulated sides. A fasciole passing beneath the anus and continued on the sides. 

 Apical disk elongated and composed of four perforated genital and five perforated ocular 

 plates. Tubercles perforate, their bosses crenulate. No dental apparatus.] 



Diagnosis. C. ambitu ovato-cordato, dor so elevato declivente, sulco 

 antico profundo sub-verticali ; vertice vaJde excentrico ; extremitate ajiali 

 truncato^ imminente. 



Synonyms. Spatangus excentricus, Rose, in Woodward's Geology of 

 Norfolk, p. 27, pi. 1, f. o. (1833.) Cardiaster excentricus, Forbes, Ann. 

 Nat. Hist. 2d series, vol. vi. p. 443. 



This singular sea-urchin is of an oblong outline, truncated behind ^ 

 deeply notched by the anteal sulcus in front, and having its widest 

 portion in front of the anterior third of its width. The dorsal sur- 

 face is very high anteriorly, the walls of the anteal sulcus rising 

 nearly perpendicularly to the highest part of the body immediately 

 in front of the apical disk. From the point of its greatest eleva- 

 tion the back gradually declines in a nearly straight line, forming 

 a long sub-carinated ridge, until it reaches the posterior truncated 

 extremity at about half the height of the entire body. On each 

 side of the centre of the back the sides fall away steeply, but the 

 margins of the body are rounded, and, in the antero-lateral regions, 

 tumid. The posterior truncation in some specimens inclines at 

 a gentle angle. The under surface is gently rounded and some- 

 what convex. 



The anterior ambulacrum is lodged in a deep sulcus with sharply 

 angulated sides. The bottom of this furrow is rounded, and covered 

 with minute granules but no tubercles. The ambulacral pores are 

 inconspicuous ; they become close-ranked in the upper portion- 

 The uppermost portion is reflected on the back with a rapid curve, 

 and includes about six of the vertical rows of pairs of pores. It 



[IV, X.] 4 L 



