100. . Dr. T. Wright on Fossil Echinoderms 
the antero-lateral ambulacra, }4ths of an inch in length, are 
nearly straight, having only very slight f-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 33ths 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 peripetal fasciole closely surrounds 
the posterior ambulacral pair, makes two angles on their sides, 
and crosses to the anterior pair, where it in lke 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, and the sternal portion of 
the interambulacrum is likewise covered with a regular tubercu- 
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 
ee 
