FROM THE LOWER GREENSAND. 289 



and obliquely truncated posteriorly. Upper surface convex, elevated, and declining 

 towards the anterior border ; base flat, with peristomal depression. Sides rounded and 

 inflated, ambulacral summit excentral, and posterior ; periprocte oval, situated high up in 

 the border. 



Dimensions. — Antero-posterior diameter one inch and i^jths ; height^ one inch ; 

 transverse diameter, one inch and 2^ths. 



Description. — The test is sometimes cordiform, tapering slightly from the anterior 

 third to the posterior border ; in other specimens it is oval, its greatest diameter being 

 about the middle of the ambitus, from whence it diminishes anteriorly and posteriorly. 

 The anteal sulcus is shallow, and the pairs of ambulacra are unequal in length and not 

 much depressed (fig. 1 a). 



The antero-lateral pair are only partially petaloid, they are long, wide, and depressed 

 in their upper third, are arched above and divergent below. The poriferous zones are 

 very unequal, the anterior are very narrow and composed of small, oblong, almost equal 

 pores, each pair being disposed obliquely circumflex (fig. 2 c). 



The posterior are much larger, and formed of elongated pores, of which the external 

 are nearly twice as long as the internal series, and there are about 36 pairs of pores in 

 the petaloid portion of each row. The postero-lateral ambulacra are much shorter and 

 more symmetrical in structure, and their poriferous zones are nearly equal ; there are 

 about 16 pairs of pores in the petaloid parts of each row. The anterior single 

 ambulacrum is lodged in a shallow anteal sulcus, and is wider than the antero-lateral pair. 

 It preserves a nearly uniform width throughout, and is composed of small plates, which 

 become gradually contracted vertically throughout its upper half (fig. 2 b), a character 

 which appears to be generic rather than specific in all the Enallasters that have passed 

 through ray hands. 



In the lower half of the areas the plates are large, square, or oblong ; they are each 

 perforated near their lower and outer corners by a pair of minute approximated pores. 

 In the upper half of the area the plates become narrow, and the poriferous zones are here 

 very conspicuous ; each pair of pores is set obliquely in a kind of circumflex manner with 

 a prominent tubercle marking the divisional partition between them (fig. 2 b), a singular 

 character which was not well shown in the figured specimen, although it is well marked 

 in others in which the tubercles of the test have been preserved. The surface of the 

 plates is covered with minute granulations, closely set together in transverse rows. 



The apical disc is small and compact, the four genital plates are perforated, and the 

 right antero-lateral extends into the middle of the disc, with the madreporiform body on 

 its surface ; the elements of the disc are closely blended with the areal plates (fig. 2 b). 



Peristome subpentagonal and lodged in a depression ; it is encircled by a smooth 

 border, and is elongated transversely (fig. 1 b). 



The periprocte is small and oval, and opens at the summit of the truncated posterior 

 border, within a smooth, undefined, anal area (fig. 1 a and d). 



37 



