from the Island of Malta. 195 



up on the posterior border, and the mouth lies very close to the 

 anteal sulcus. 



Affinities and dijferences. — The central position of the apical 

 disc and the depth and strai2;htness of the ambulacra distin- 

 guish this form from P. excentricus ; the generic affinities of 

 this small genus have been already pointed out. The rarity of 

 Pericosmus, and the small number of species and individuals at 

 present known, limit our comparisons to the forms we have 

 cited. 



Locality and stratigraphical range. — This is one of the few 

 Urchins found in the clay bed No. 2 at Malta : we have before 

 us a specimen from Balistro, Corsica, from the miocene beds of 

 that island, sent us by M. Michelin of Paris; we consider the 

 peculiar arch-like arrangement of the peripetal fasciole, with the 

 marginal, as good generic characters whereby to form a distinct 

 genus. 



Pericosmus excentricus, Wright, n. sp. 



Test oblong, highly convex above, slightly so below ; apical disc 

 very excentral, near the anterior border ; ambulaci'a in shallow 

 depressions ; single ambulacrum slightly grooves the anterior 

 border ; antero-laterals nearly transverse ; postero-laterals in- 

 cline at 55°; peripetal fasciole narrow and undulating; mai'- 

 ginal fascioie narrow and low on the border ; tubercles on the 

 upper surface small, close-set, and nearly all of the same size ; 

 a few larger ones on the anterior interambulacra ; anus lai'ge 

 and situated high on the border; mouth-opening wide in the 

 anterior third, surrounded by five poriferons petaloid zones; 

 sternal portion of the interambulacrum convex, with close-set 

 imbricated tuberculigerous plates ; basal portions of the inter- 

 ambulacral pairs with larger tubercles, wider apart, and more 

 irregular than those of the dorsum. 



Dimensions. — Antero-posterior diameter 2 inches, transvei'se 

 diameter 1 -^-^ inch, height 1 inch ? 



Description. — This Urchin is so much crushed that it is im- 

 possible to describe its outline. The ambulacral areas form 

 shallow depressions, and the single area slightly grooves the an- 

 terior border : the antero-laterals are nearly transverse, their in- 

 clination being forwards ; they are y^ths of an inch in length : 

 the postero-laterals form an angle of 55° ; they are y^ths of an 

 inch long. The crushed state of the test makes it impossible to 

 count accurately the pores, or give the breadth of the areas ; the 

 apical disc, with four genital pores, is very small and remarkably 

 excentrical, being very near the anterior border ; the peripetal 

 fasciole is narrow, angular and undulating, and instead of sur- 

 rounding the anterior part of the antero-lateral ambulacra, as in 



13* 



