116 PAPERS, ETC. 
where they are the most abundant species, and with them 
are five new species, described in this paper. Not less 
than nineteen liasic and oolitie species are now known. 
These shells are in general attached to other bodies; and, 
as their forms are modified from this circumstance, greater 
care is necessary in the determination of species ; and more 
especially as the same species presents great contrast in 
form, depending upon age and the state of perfection in 
which the shell is found. This may be seen on comparing 
T. serratum, fig. 3, plate 3 (in which the supra-mem- 
braneal disk is preserved), with figs. 4—5, which are 
more imperfect forms of the same species. The same may 
be noticed on comparing 7. Forbesei, fig. 9, plate 3, with 
fig. 10. 
THECIDEUM BOUCHARDIL— Dav.—Plate 1, fig. 11—13. 
Shell inequivalve, flattened, sub-eireular; attached by 
the prineipal portion of the ventral valve; deltideum 
large, elevated, triangular; area large and extended, shew- 
ing lines of growth; hinge line depressed in centre, leaving 
a small flat area under the deltideum ; dorsal valve much 
smaller than the ventral. The interior of’the ventral valve 
shews a slight middle septum, on each side of which are 
two large scars, due to the attachment of the cardinal 
muscle, on the outer edge of which are two small depres- 
sions, which received the adduetor muscles ; interior 
rugosely striated; the cavity of the valve in adult shells 
surrounded by a broad margin, having a wavy appearance, 
due to lines of growth. Interior of dorsal valve has a 
broad granulated margin, within which is a very high cen- 
tral septum, nearly reaching the surface of the opposite 
valve, from whence proceeds a granulated ridge, united by 
