i858 
existence depend on the neat preservation of the specimens, 
being chiefly in a stratum of little else than clean fragments 
and minute shells of their own texture, which preserve them 
by defending them from the ruder fragments and change of 
weather? 
As the sides are usually broken off, I have had recourse 
to several specimens to produce the upper figure, which I 
hope exhibits the character tolerably correct. The shell 
is soincorporated with the stone, that it is not possible to 
detach it and expose the hinge: it is often partially filled 
with, foliated Carbonate of Lime, exhibiting the diagonal 
fracture. Fig. 2 gives two views of a specimen that appears 
to have been distorted when the rock was in such a soft 
state as to assimilate with the shell, and both were com- 
pressed or bent together, a circumstance characteristic of 
the Irish Black Rock. 
CARDIUM elongatum. 
TAB. LXXXII.—Fig. 3. 
Sprc. Cuar. Ovate, ventricose, longitudinally and 
obtusely plaited ; anterior side elongated, plain ; 
posterior side produced. 
Syn. Arcites rostratus. Mart. Petr. Derb. pl. 44. 
Txcuupina the elongated sides, this shell is twice as wide 
as long; the longer side is nearly cylindrical and free from 
strie; the beaks are rounded. 
This is from Derbyshire, and is the same species as 
figured by our late friend Mr. Martyn as an Arca, which 
it was generally thought to resemble. Its relation to the 
Genus Cardium will not, however, now be doubted, having 
seen the Cardium hibernicum. How near the rock con- 
taining this in Derbyshire is related to the black rock of © 
Ireland, will by degrees be ascertained. Some of the 
'Terebratulee which accompany them are very similar, if not 
exactly alike in both rocks, agreeing with Anomia striata of 
Martyn. This specimen was among others presented to me 
by Mr. Martyn before he begun his work, and I presume it 
is from the place where his specimen was found. 
