from the Island of Malta. 99 
Genus Pericosmus (Agassiz, 1847). 
In addition to the general characters of Hemiaster, these 
Urchins have an arched arrangement of the peripetal fasciole and 
a narrow marginal fasciole, which can be traced round the 
anterior border, extending along the sides, passing beneath the 
anus, meeting its fellow from the opposite side, and thereby 
encircling the test. All the species of this small group have 
been obtained from strata of the Miocene age. 
Pericosmus latus, Desor. 
Syn. Micraster latus, Agassiz, Cat. Syst. p. 2. 
Pericosmus latus, Agassiz and Desor, Cat. raisonné, Ann. Se. Nat. 
tom. vi. pl. 16. fig. 1, & tom. viii. p. 19. 
Test cordate, broad, convex above, flat below; petaloidal ambu- 
lacra straight, deep-sunk and narrow; the posterior nearly 
as long as the anterior pair; apical discs central; peripetal 
fasciole closely embracing the ambulacra, with three arches 
across the single ambulacral depression ; marginal fasciole 
narrow, entirely surrounding the upper part of the border of 
the test. 
Dimensions.—Antero-posterior diameter 2,7, inches; trans- 
verse diameter 2,7, inches; height 1,3; inch. 
Description.—This rare type of one of the extinct genera of 
Spatangide was at first mistaken for a Micraster by Agassiz, and 
entered in his ‘Catalogus Systematicus’ under the name Mi- 
eraster latus ; the peripetal fasciole, however, readily distinguishes 
it from Micraster, and the marginal fasciole from Hemiaster. No 
doubt many mistakes will be committed regarding this Urchin, 
as these fascioles are exceedingly delicate, and not always pre- 
served: when they are absent, it then greatly resembles a Mi- 
craster ; but when the marginal fasciole is effaced, and the peri- 
petal remains, it then may be mistaken for a Hemiaster; for- 
tunately, in one of the specimens before us, the fascioles are both 
preserved, and their entire course can be traced. The test has 
a uniformly curved dorsal surface, with a regular cordate form, 
the base is slightly convex, and the posterior border is trun- 
eated. The petaloid portions of the ambulacral areas are 
straight, and lodged in rather deep depressions, extended ob- 
liquely outwards on the middle of the dorsal surface, in the form 
of a St. Andrew’s cross; the single ambulacrum lies in a deep 
wide depression, and forms a considerable anteal sulcus; at its 
apical portion only, there are from ten to twelve pairs of ambu- 
lacral plates, each perforated in the centre with a single hole ; 
in all the other plates in the area the perforations are obsolete ; 
