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. raisonne, Ann. Sc. Nat. 

 torn. vi. pi. 16. fig. 1, & torn. 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 2fo inches; trans- 

 verse diameter 2/ n inches ; height l T 3 n inch. 



Description. This rare type of one of the extinct genera of 

 Spatangida was at first mistaken for a Micraster by Agassiz, and 

 entered in his ' Catalogus Systematicus ' under the name Mi- 

 craster 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 fascicles 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- 

 cated. 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 ; 



