378 CLYPEUS 



Clypeus Agassizii, Wright. PL XXXI and XXXII. 



NucLEOLiTES Agassizii. Wright, Ann. and Mag. of Nat. Hist., 2d series, vol, ix, p. 308, 



PI. iii, fig. 3 a—c, 1851. 

 — — Forbes, in Morris's Catalogue of Brit. Foss., 2d ed. p. 84, 1854. 



Clypeus Agassizii. Desor, Synopsis des Echinides Fossiles, p. 278, 1857. 



— — Davoust. Note sur les Fossiles speciaux a la Sarthe, p. 25, 1856. 



— — Cotteau and Triger, Echinides du departement de la Sarthe, pp. 



16 and 61, pi. iii, fig. 1 ; pi. ix, fig. 9 ; pi. ix, fig. 1 — 3. 



Test large, hemispherical, or sub-conoidal ; margin sub-circular ; a little longer in the 

 antero-posterior diameter; rounded before, slightly rostrated, and truncated behind; 

 vertex central, apical disc excentral, and inclined backwards ; ambulacral areas very 

 narrow ; poriferous zones widely petaloid and conjugate from above the margin to the disc ; 

 at the margin and base, pores simple, non-conjugate, and scarcely visible ; near the mouth 

 more numerous and apparent, and arranged in triple oblique pairs ; vent oblong, and 

 near the surface; anal valley very shallow, commencing in the middle of the single 

 inter-ambulacrum ; an undepressed portion of test between the vent and disc, sometimes 

 sulcated in the middle ; base flat, or slightly undulated ; mouth-opening large, subcentral ; 

 peristome surrounded by five prominent lobes ; dorsal tubercles nearly microscopic ; basal, 

 a Httle more conspicuous. 



Dimensions. — Pleight, two inches and one quarter; antero-posterior diameter, four 

 inches and one fifth ; transverse diameter, four inches. 



Description. — The large size, the hemispherical or sub-conoidal form of the upper 

 surface, the flat base, and superficial anal valley, readily distinguish this magnificent species 

 from all other Clypei. The outline of the margin is nearly circular, the antero-posterior 

 being a little more than the transverse diameter ; the test is rounded before, and slightly 

 rostrated, and truncated behind ; the upper surface assumes a conoidal figure (PI. XXXII, 

 fig. 1, a) ; the vertex is central, and the test declines more towards the posterior than the 

 anterior border. 



The ambulacra are long, narrow, and lanceolate ; the anterior area is straight ; the 

 antero-lateral pair describe an /-shaped curve on the sides of the test (PL XXXII, fig. 

 1, a) ; and the postero-lateral curve forwards upwards and inwards at the posterior surface 

 (PL XXXII, fig. 1, b). 



The poriferous zones are very large, and widely petaloid on the upper surface (PL 

 XXXI, fig. 1, a); they are formed of an inner row of small, simple, round, or oval pores, 

 which extend equidistant from the margin to the disc, and an external row, of long. 



