352 ECHINOBRISSUS 



the posterior pair are one third wider than the anterior pair, and the inter-ambulacrum is 

 the widest. There are about sixteen plates in each postero-lateral column, and the surface 

 of each is covered with numerous small tubercles (fig. 3 d), arranged in five or six 

 horizontal rows. 



The anal valley extends about two thirds of the distance from the posterior border to 

 the apical disc ; it is a narrower and more shallow sulcus than the corresponding talley in 

 U. scutatus, and the vent opens nearer the surface. Between its upper border and the 

 disc there is a well-marked triangular space, in length about one third the distance between 

 the disc and border ; the base of which is formed by the arch of the sulcus and the 

 lateral portions of undepressed test, and the sides, by the long, oblique, posterior 

 poriferous zones (fig. 3 a) ; this space is undepressed, and the plates composing it differ 

 in no respect from the plates occupying the same region in other areas. When viewed 

 laterally, the test of U. dimidiatus forms a long gentle slope from the vertex to the posterior 

 border (fig. 3 c), and a short abrupt slope towards the anterior border ; this arises from the 

 excentral position of the apical disc, and the greater height of the anterior portion of the test. 



The base is concave towards the centre, and tumid at the sides ; the small mouth- 

 opening is situated opposite the apical disc ; the peristome is pentagonal, and its angles 

 correspond to each of the ambulacral areas ; the tubercles at the base are larger and more 

 closely crowded together than those on the upper surface ; the poriferous zones are so 

 indistinct at the base that they appear only as faint lines radiating from the peristome. 



The apical disc is very small, the madreporiform body makes a slight pyriform 

 prominence, and the pairs of perforated genital plates extend outwards between the 

 ambulacral areas. 



Affinities and differences. — In its general outline this species resembles E. Goldfussii, 

 Desor, from the Kelloway ferrugineux of the Sarthe ; in that species, however, the vent is 

 nearer the border, and the test proportionately higher to its length. It so closely resembles 

 E. scutatus, Lamk., that I formerly considered it a variety of the latter. It is, however, 

 distinguished from E. scutatus by the following characters : the test is more elongated ; 

 the sides and base are more tumid ; the apical disc is smaller and more excentral ; the anal 

 valley is smaller and shallower, and rarely extends so high up the area ; the poriferous zones 

 are wider, and their petaloid portions lie in slight depressions on the upper surface ; the 

 sides are very tumid, and they are frequently more irregular in outUne and unsymmetrical 

 in proportions than in E. scutatus ; the base is more concave, and its sides more tumid ; 

 the slope of the dorsal surface likewise, from the disc to the posterior border, is longer 

 and more inclined than in its nearly allied E. scutatus. 



Locality and Stratigrapliical position. — E. dimidiatus, Phil., is collected in considerable 

 abundance from the Coralline Oolite at Malton, Yorkshire, where it is associated with 

 E. scutatus, Lamk., Clypeus subulatus, Young, and Fygurus giganteus, Wr. The finest 



