100 Dr. T. Wright on Foss-il Echinoderms 



the antero-lateral ambulacra, ^ths of an inch in length, are 

 nearly straight, having only very slight /-shaped flexures, which 

 curve forwards and outwards, making an angle of 35 : the two 

 poriferous zones lie close together, in deep narrow depressions ; 

 in each zone there are from twenty-four to twenty-six pairs of 

 holes ; the space between the pairs of pores is only a little more 

 than that which separates one row of pores from another, so 

 that the pores lie nearly equidistant from each other in the 

 sunken areas; the postero-lateral ambulacra are fifths of an 

 inch in length, and are extended backwards and outwards at an 

 angle of 60 ; there are from twenty to twenty-two pairs of holes 

 in each zone, the rows of which, like those on the anterior pair, 

 are nearly equidistant : the pcripetal fasciole closely surrounds 

 the posterior ambulacral.pair, makes two angles on their sides, 

 and crosses to the anterior pair, where it in like manner forms 

 two angles, then sweeps round the base, and passes in a straight 

 line along the anterior side ; from it three branches proceed in- 

 wards, which arch over the single ambulacrum, describing angles 

 as they advance to meet the branches from the opposite side ; 

 the marginal fasciole is a narrow line, which passes above the 

 fold of the border and entirely encircles the test, dipping into 

 the anteal sulcus in its course, but its position in relation to the 

 anus is not clearly shown in either of our specimens. Agassiz 

 figures it as passing under the anus ; in his figure the remark- 

 able arches on the anterior part are not drawn : the apical disc 

 is small, and occupies the centre of the test ; it has only two 

 genital holes like a Hemiaster, and five small ocular holes ; the 

 upper surface of the test is covered with small, nearly equal- 

 sized tubercles, those on the anterior side are a little larger; 

 the base is slightly convex, the lateral interambulacra carry 

 large tubercles on their basal plates, arid the sternal portion of 

 the interambulacrum is likewise covered with a regular tuberc il- 

 lation ; the basal tracks of the posterior ambulacra are smooth 

 between the border and the mouth ; the anus is situated high 

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

 anteal sulcus. 



Affinities and differences. The central position of the apical 

 disc and the depth and straightness 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 



