62 EOCENE MOLLUSCA. 
MODIOLA. Lamarck, 1801. 
_ Generic Character. Shell equivalve, inequilateral, irregularly and roundedly oblong or 
trapezoidal ; valves sometimes smooth, sometimes entirely covered with radiating strie, at 
others the central portion is smooth, with the lateral extremities striated ; pedal region small, 
umbones subterminal ; hinge-margin generally smooth, sometimes crenulated, edentulous ; 
connexus bipartite; impressions of the adductors unequal; shell slightly gaping for the 
passage of a byssus; epidermis in the recent state often produced into long, beard-like 
fringes ; interior of shell nacreous. 
Animal with the margins of the mantle without fringe; foot cylindrical, clongated ; 
spins generally a fine and ample byssus. 
This is, as it were, an emanation from the last, with a further approach towards the true 
Dimyaria. Unlike the preceding, in which the umbo is pointed and terminal, the animals 
of this genus extend their shells beyond the beak on the pedal side, altering their form 
from the wedge-shape of Mytilus to the sub-rhomboidal shape of this. In I/ytilus the 
oral adductor is immediately beneath the umbo, but in this genus it is beyond it, and on 
the inner side of the cartilaginous portion of the connexus there is generally a deeply 
impressed mark of the pedal muscle. 
The habits of these animals in the living state are variable; many of the species spin a 
byssus, by which they are constantly fixed, and this byssus in some is so enlarged as to 
envelop the shell in a kind of nest; others bore into the test of an Ascidian; while for 
some cylindrically formed shells, such as IZ. Uithophagus, Linn., a habitation is excavated 
in corals, shells, and the hardest limestone rocks; these latter, from such habits and 
their cylindrical form, have by some naturalists been considered as entitled to a distinct 
generic position (Lzthodomus, Cuv.). These boring shells are found in the Oolites, in the 
thick shells of Z)ichites and Astarte, as well asin the rock itself, and shells resembling them 
have been met with in the Palaeozoic formations. 
Crenella is another section of this genus which has been put forward as a claimant for 
isolation; the principal, perhaps the only, distinction is the striation of the exterior, but 
this in itself is insufficient. Shells included in the above diagnosis possess every possible 
variation. In some the shell is quite smooth or naked, but in others it is less than half orna- 
mented, increasing the extension in striation until many are entirely covered ; while some 
have the centre smooth, with the extremities striated. This, again, is in one species 
reversed, the striz only occupying the central portion. 
Myopara is a name proposed as a genus by Lea in substitution of Stalagiivm, Conrad, 
1833 (Morton’s ‘Synopsis,’ App., p. 8), for the reception of a small Eocene fossil of 
America, which is possibly an aberrant form of this genus, as suggested by Mr. Wood- 
ward, belonging to the section Crenedla. Itis of a more ovate outline than are the generality 
