66 



Dr. T. Wright on Fossil Echinoderms 



belong to what we regard as the typical form. This remark- 

 able exceptional specimen agrees with the brief notice of C. 

 Tauricus* y Desor: " Tres grande espece, allongee, pentagonale, 

 a bord fortement renfle. Zones poriferes tres large k leur ex- 

 tremite. Tert. du Taurus, ile de Crete." If we are correct in 

 referring all these varieties to C. altus, it follows that this species 

 has a wide range of deviation from what we take to be its typical 

 form ; but these limits of variation are probably not greater in 

 this than in some other species of Urchins. The following 

 table shows the relative dimensions of three forms, the typical, 

 the conical, and the flattened and tumid varieties : 



The ambulacral areas are largely petaloid, nearly equal in 

 length and width ; they are rounded, widely open below, 

 and extend over four-fifths of the dorsal surface in the type 

 form, over nearly three-fourths in the conical form, and over 

 almost four-fifths in the tumid varieties ; in all, the areas form 

 prominent convex elevations of the test, which are bounded 

 by wide poriferous avenues, composed of two series of simple 

 pores united by oblique grooves ; the internal series of pores are 

 round, the external series are elongated transversely in the 

 direction of the grooves ; the pores at the end of the avenues 

 are much more so than those of the summit ; the apical disc 

 is small, and occupies the centre of the dorsal surface, lying in 

 a slight depression formed by the bending-in of the summits of 

 the areas ; those of the single area, and the antero-lateral and 

 postero-lateral areas of the left side, being rather more pro- 

 minent than those of the right side of the test. The madre- 

 poriform body occupies the whole surface of the disc, the ocular 

 and genital plates being quite indistinguishable from the general 

 structure of the test ; the five genital pores pass obliquely into 

 the interior, at a short distance from the disc. 



The base is flat, and the mouth lies in a very deep depression 

 in the centre of the under surface ; the opening is pentagonal, 

 its wide walls being formed by the incurving of the basal por- 

 tions of the areas ; from each of the angles of the pentagonal 

 opening, a deep furrow passes outwards towards the margin of 

 the test, and becomes continuous with the median suture of the 



* Cat. raisonne des Echinides, Ann. Sc. Nat. torn. vii. p. 131. 



