from the Island of Malta. 179 



of moderate size ; basal portions of the postero-lateral ambu- 

 lacra form two smooth tracks destitute of tubercles between 

 the posterior border and the mouth. 



Dimensions. — Antero-posterior diameter 1 1% inch ; transverse 

 diameter ly\ inch ; height at the interambulacrum ^ Jths of an 

 inch. 



Description. — An imperfect specimen of this Urchin was at 

 first mistaken for a small variety of Spatangus Desmarestii, 

 Goldf., and entered under that name in the list of fossils from 

 bed No. 2. Having lately obtained a better specimen of this 

 form, we are now enabled to give a description of it, which will 

 be found to differ in many essential points from that species, to 

 which it was at first referred. The test is regularly cordate, 

 slightly flattened at the cheeks, bulging out at the sides, and 

 from thence tapering abruptly towards the posterior border, 

 where it is truncated obliquely downwards and forwards; the 

 anterior part of the test is flattened and depressed, and the pos- 

 terior portion much elevated, from the circumstance of the single 

 interambulacrum forming two prominent ridges, one on the 

 dorsal, and the other on the basal surface, which gives increased 

 depth to the test, tilts it up, and forms an inclined plane of the 

 dorsal surface. The petaloidal ambulacra are short, broad and 

 leaf-like ; the anterior pair are slightly flexed forwards and form 

 an angle of about 15° ; their anterior poriferous zone is nearly 

 as complete as the posterior zone, which contain respectively 

 sixteen and eighteen pairs of pores ; the posterior pair form two 

 oval leaflets, the bases and apices of which are nearly equally 

 curved, and closed with pores, having about eighteen pairs of 

 pores in each zone ; they describe an angle of about 60^. The 

 single anterior ambulacrum makes a very slight depression on 

 the upper part of the anterior region, and the anteal sulcus 

 formed by it is inconsiderable when compared with other con- 

 generic forms ; the anterior and lateral pairs of interambulacra 

 carry a few large crenulated and perforated tubercles on the 

 upper part of their dorsal surface ; on the anterior pair there are 

 from seven to eight, and on the posterior pair from four to five 

 of these tubercles, which are neither so large nor yet have such 

 deep areolas as their homologues in >Si. Hoffmanni. The single 

 posterior interambulacrum is narrow, but greatly developed in 

 the vertical diameter ; above, a blunt prominent ridge commences 

 near the apical disc and extends to the posterior border ; below, 

 another ridge commences a short distance from the truncated 

 portion of the border, and extends to the centre of the mouth ; 

 the base is slightly convex from side to side, besides being 

 raised in the middle of its posterior part by the ridge just 



12* 



