from the Island of Malta. 89 
rior border; sternal portion of the interambulacrum with a 
regular ornamentation. The subanal fasciole very near the 
anus is heart-shaped and narrow ; it encloses rows of tubercles 
which are arranged in radii in regular order; before the 
fasciole the test forms a projection, and from the summit 
thereof, rows of tubercles arranged in straight lines extend 
towards the mouth, increasing in size as they approach that 
opening; the basal portions of the other mterambulacral 
areas are covered with scale-like imbricated plates, each car- 
rying an oyal eminence with a crenulated summit, and a 
tubercle placed at the anterior side of the oval eminence ; 
these tubercles are all regularly arranged in rows which have 
a direction forwards and outwards : the postero-lateral ambu- 
lacra form a naked space, which separates the imbricated basal 
portions of the pairs of interambulacra from the ornamented 
sternal portion of the single one. The anus is large and 
situated at the posterior border ; both this opening and the 
mouth are much injured. 
Dimensions.—Antero-posterior diameter 3%, inches, transverse 
diameter 3 inches, height ,8,ths of an inch. 
Description —The detailed diagnosis given of this species con- 
tains nearly all that we can describe of this Brissus, for, with the 
exception of a small portion of its anterior part preserving a por- 
tion of the peripetal fasciole, all the rest is absent; the regu- 
larity in the arrangement of the tubercles at the base constitutes 
a characteristic feature of this form, and the imbricated style of 
the basal plates, resembling the tegumentary membrane of a 
placoid fish, gives value to the specific name. ; 
Affinities and differences.—The order and symmetry of the de- 
coration of the sternal portion of the interambulacrum, the 
heart-shaped subanal fasciole, with its broad band of microscopic 
granules, and the leaf-like tuberculated expansion which extends 
from the apex of the fasciole, are very characteristic of this spe- 
cies; if to these we add the imbricated style of the plates occu-. 
pying the sides of the base, and the oblique way the tubercles 
are set on their oblong bases, we have an assemblage of organic 
characters by which B. imbricatus may be readily distinguished 
from its congeners. The form of the test, the size of the 
tubercles, the symmetry of the subanal rosette, formed by radii 
of tubercles, and encircled by a broad fasciole, readily separate 
it from B. latus, with which it is associated in the same stratum. 
Locality and stratigraphical position.—This species was col- 
lected from bed No. 1, the Gozo marble, at Malta : it is the pro- 
perty of the Bristol Institution. 
H2 
