from the Island of Malta. 193 



Affinities and differences. — Tlie absence of the dorsal carina 

 and caudal process serve to distinguish //. Scillce from //. Cot- 

 teauii; the latter is likewise a more globose and less elegant form 

 of Urchin, and has the truncature of the posterior border down- 

 wards and forwards, whereas in H. Scilla the direction is down- 

 wards and backwards. 



Localifij and stratiyraphical position. — Collected from No. 4, 

 the calcareous sandstone at Malta, from whence the original spe- 

 cimen figured by Scilla was obtained : this reason will suffice for 

 the name we have given it. 



Genus Pericosmus (Agassiz, 1847). 



In addition to the general characters of Hemiaster, these 

 Urchins have an archt-d arrangement of the ])eripetal 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 2^^ inches; traus- 

 veise diameter 2/^ inches; height ly'^ inch. 



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

 Spatangidce 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 flemiasier. 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 mai'ginal 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 



Ann. ^ Mac/. N. Hist. Ser. 2. Vol. xv. 13 



