204 THE FOSSIL ECHINOIDEA 



sunken in small cavities, which simulate a uniserial line of single pores, placed wide 

 apart. 



The antero-lateral petals are situated in wide and rather deep cavities ; they are 

 widely divergent, subtending an angle of about 120°, and have a slightly arched 

 appearance, in consequence of the curvature of the anterior zone, which is bent con- 

 vexly forwards, whilst the posterior zone is comparatively straight. The posterior petals 

 are rather longer than the anterior pair, they are slightly curved, with concavity 

 forwards and outwards, and the inner margins of the two petals are very near together 

 for quite half their length. The odd posterior interradial space is a narrow wedge- 

 shaped strip, which presents scarcely any tumidity on the inner part of the area ; the 

 character developing gradually, however, and culminating in the region external to the 

 petals. The cavities in which the posterior petals are lodged are scarcely so deep as 

 those of the anterior petals ; and the curvature of the respective petals gives a decided 

 suggestion of a semicircular arch formed by the union of the anterior and posterior 

 lateral petals. 



The poriferous zones of the petatoid ambulacra have their pores large, oval or 

 comma-shaped, with the pores of a pair rather wide apart and united by an indistinct 

 groove; a rather broad strip occurs between neighbouring pairs, but no keel is formed, 

 though its position is occupied by a single row of small uniform granules borne on the 

 aboral margin of the ambulacral plate. The interporiferous area is extremely narrow, 

 its width being less than the distance which separates the pores of a pair. The four or 

 five pairs of pores near the apical extremity of the anterior zone of the antero-lateral 

 petals, and the seven or eight similar pairs of the posterior zone of the postero-lateral 

 petals are much aborted and reduced in size. 



The posterior ambulacral areas on the actinal surface are very wide and naked, and 

 they bound the prominently keeled actinal plastron. 



The peristome is large and excentric in front, its position being about equidistant 

 from the anterior and lateral margins of the test. The anterior margin of the aperture 

 is semicircular, and the posterior lip is prominent and more or less keeled. 



The periproct is very large, subcircular in form, and situated at the summit of the 

 posterior truncated extremity. The inferior portion of this extremity is slightly 

 depressed beneath the periproct, forming a faint concavity which gives emphasis to 

 the rather abrupt separation from the subanal plastron, the plane of which stands at an 

 angle between the anal plastron on the one hand and the posterior extremity on the 

 other. The ornamentation of the concavity beneath the periproct is devoid of any 

 large tubercles. The ornamentation of the subanal plastron is very small on the side 

 adjacent to this concave area, and increases gradually in size as it recedes from it 

 and approaches the actinal plastron. 



Two fascioles are present on the test, the peripetalous and the subanal. The 

 peripetalous fasciole is very sinuous and fiddle-shaped ; it crosses the anteal sulcus with 

 a wide sweep just above the rounding of the margin, traverses the central portion of the 

 anterior interradia obliquely, and on the posterior column of this area is sharply bent 

 inwards nearly at a right angle, but is almost immediately again abruptly deflected, 



