16 THE TERTIARY FOSSIL ECHINOIDEA 



1. Ambltpygus altus, Bimcan & 8laden. Plate IV, Figs. 1-3. 



The test is large, very thick, swollen, and subhemispherical above, tumid at the 

 margins and on the actinal surface also. Marginal contour nearly circular. Abactinal 

 surface sloping nearly equally to the front and back, but it is slightly more swollen 

 posteriorly than anteriorly. The lateral slope approaches the curve of a semicircle, but 

 is rather flatter in the upper one third. The posterior interradium is more flattened 

 than the others at the margin. Apical system slightly in front of the centre or central ; 

 the system is small in relation to the dimensions of the test, and is pentagonal or irre- 

 gularly circular in outline. The generative pores are large and stand outside of the 

 irregularly-shaped madreporic body, vv^hich separates the posterior ocular plates rather 

 widely. 



The anterior ocular plate is distinct, and the pore is well in front of the anterior 

 edges of the antero-lateral generative plates ; these are large, and the right one is 

 largely covered by the madreporic, and the left one much less so. The antero-lateral 

 ocular pores are sunken, and their plates are well developed. The postero-lateral gene- 

 rative plates are wide apart, and separated by the low, tumid, madreporic body ; their 

 pores are large. The madreporic body has an irregular margin, in no way overlaps the 

 pores, and has its surface covered with close, small, low granules, the intermediate 

 surface being minutely pitted. 



The ambulacra are flush with the test, are largely open just p,bove the margin 

 where the poriferous zone alters its character, and become narrower towards the peri- 

 stome. The poriferous zones of each ambulacrum converge high up in the peristome 

 on its ascending process. 



The poriferous zones are well developed abactinally, increase in breadth rather 

 rapidly from the apex, and then diminish more gradually to close above tbe margin, 

 where the pores are smaller, those of each pair coming nearer together and being more 

 obliquely placed. 



The long inner row of pores, circular in outline and well open, forms a nearly straight 

 and diverging linear series; and the outer pores, which are elongate transversely and in the 

 shape of long ellipses, form a curve in vertical series. The pores are conjugate, but the 

 groove is shallow and broad, and usually slightly bent. A low, broad, costal projection 

 is between each succeeding groove, and has on it, where the zone is widest, about 7 

 distinct granules in a line, with alternate ones smaller than the others. Near the margin 

 the pores of each pair are not on the same level, the inner one being the lowest down ; 

 and on the margin this obliquity becomes very decided, the outer pore being nearly 

 above the inner one^ and separated by a narrow and linear bridge. The pairs increase 

 in number at the margin and over it, and become fewer near the peristome, where 

 they again become less oblique. The pores pass up within the peristome on its vertical 

 waUs and end close to their free edge. There is no phyllode. Every third pair of pores 

 is in the centre of a small accessory plate. The anterior odd ambulacrum has a curved 

 course actinally, and is not placed there in a line with the longitudinal axis of the 

 test. 



