NEW SPECIES OF HOLASTER. 365 



flattened, but rounded at the border. Mouth opening in a slight 

 depression near the anterior border ; anal opening above the base 

 on the posterior border, with a small depression or hollow below it. 

 The dimensions of full-grown specimens are two and a half inches 

 in breadth or transverse diameter and the same in length ; height 

 about two inches. The ambulacral summit is central ; the ambu- 

 lacra, which are straight, lanceolate, and apetaloid, radiate from it. 



The anteal sulcus is feebl}' developed, the depression in front of 

 the mouth being continued for a short distance along the ambulacral 

 area, but hardly far enough to break the circular outline when the 

 test is viewed from the summit. 



Both the antero- and postero-lateral ambulacra are straight and 

 nearly equal ; the poriferous zones are similar and narrow ; the 

 pores are clearly visible from summit to base, about 50 being 

 visible in each avenue of the anterior pair, but not quite so many 

 on the posterior pair. The apices of the three anterior pairs converge 

 at a little distance from the posterior pair, just as in Holaster 

 subglohosus. 



The composition of the apical disk appears to be as usual in 

 the genus Holaster, and the madreporiform body is distinct in the 

 specimens examined. 



The interambulacral areas are formed of large wide plates, those 

 between the lateral ambulacra being more than three times the 

 width of the ambulacral plates. Their surface appears to have 

 been minutely granulated and ornamented with primary tubercles 

 rather sparsely and irregularly scattered. 



Affinities and differences. — The characters of this well-marked 

 species are distinctly those of Holaster, but some of them serve to 

 connect this genus very closely with Ecliinocorys (Ananehytes). Its 

 general shape is, indeed, so similar to the more globular forms of 

 Echinocorys vidgaris that the imperfect specimens obtained by the 

 fossil-collector of the Geological Survey were mistaken for that 

 species, and catalogued under that name in the Survey Memoir 

 (Explanation of Sheet 84). The peculiarity of the supposed occur- 

 rence of that species in the Lower Chalk was remarked upon at p. 147 

 of that memoir. 



It is the largest of our British Holasters, though it seems to be 

 occasionally rivalled by Holaster subglohosus ; in many respects it 

 resembles this species, but differs in several important particulars. 

 In H. subglohosus the test is sometimes nearly circular, but is 

 generally more or less cordiform, narrowing towards the posterior 

 border, which is truncated somewhat abruptly, and there is generally 

 a dorsal carina ; the under surface is always convex, and in many 

 specimens the sectional curve of the sides is nearly equal upwards 

 and downwards ; the anal aperture is situated on the horizon of 

 greatest width, which is about midway between apex and base ; 

 lastly the anteal sulcus is always visible when the test is viewed 

 from the top. As compared with this, the test of H. rotundus is 

 generally circular, though in one specimen before me it is slightly 

 produced posteriorly so as to have a suboval outline and an obtusely 



