﻿396 



PYGURUS 



lanceolate. The poriferous zones on the dorsal surface are formed of an inner row of oblong 

 holes, and an outer row of oblique, slit-like apertures, both rows being conjugate through 

 very line sulci (fig. 2 b) ; at the lower fourth of the zone the slit-like outer pores contract 

 into simple pores like those of the inner row ; at the border the pairs of pores lie close 

 together, whilst at the base they are wide apart ; near the mouth-opening the ambulacra form 

 phylloidal expansions, in which the pores lie crowded in triple, oblique pairs; the mara- 

 millated character of the oral lobes occasions a considerable contraction of the areas around 

 the peristome (fig. 1 b). 



The inter-ambulacral areas on the upper surface are uniformly convex ; the anterior pair 

 are narrower than the posterior pair ; the single area, which is about the same width of the 

 latter, is produced, rostrated, and deflected (fig. 1 c), and in some varieties forms a caudal 

 prolongation ; at the under surface, which is concave, the basal inter-ambulacra are moderately 

 convex ; the postero-lateral pair and single inter-ambulacrnm are more so than the anterior 

 pair ; the anal valley is deep, with inclined sides, and the vent is large and elliptical 

 (fig. 1 b). 



The mouth-opening is small, sub-central, and forwards; it lies at the most concave part 

 of the base ; the peristome is pentagonal, and the five oral lobes, formed by the terminations 

 of the inter-ambulacra, have a prominent, mammillated character, which occasions a con- 

 traction of the ambulacra at the point where they join the peristome ; the protrusion of the 

 lobes removes the mouth itself far from the surface ; the emargination of the anterior 

 border is more conspicuous at the base, and the anteal sulcus occasions a considerable 

 depression in the anterior border. 



The tubercles are very small, and arranged in close-set rows, of which there are 

 five or six on each plate (fig. 2 b) ; the areolas are well defined ; the tubercles at 

 the base are much larger than those on the upper surface ; they are more developed at the 

 border, on the declining sides of the cushions, and near the mouth, than on the convex 

 surface of these prominences, where they are small and closely aggregated together ; the 

 tubercles here are distinctly perforated (PI. XXXV, fig. 2 y), whilst the intermediate surface 

 is covered with close-set microscopic granules. Pew portions of the specimens I have 

 seen are sufficiently well preserved to show the sculpture ; in those from the Calcareous 

 Grit it is almost always effaced, whilst in those from the Coralline Oolite it is concealed 

 by the Oolitic Coralline mud in which the most of them is enveloped. 



The apical disc is small and central (fig. 1 d) ; the madreporiform body, which is 

 round, convex, and prominent, forms the vertex, the large ambulacral petals covering 

 nearly all the other portion of the upper surface. In fig. 1 c the dorsum forms a conical 

 eminence through the development of the petals and disc (fig. 1 a). 



Affinities and differences. — This species so closely resembles Pyyurus Michclini, Cott., 

 that it is only after a careful analysis of its specific characters the differences between them 

 are discovered. Pyyurus pentayonalis is more convex on the upper surface, and more con- 



