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 striae, 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 j 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, elongated ; 

 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 My til as 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 M. lithophayus, 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 {Lithodomus, Cuv.). These boring shells are found in the Oolites, in the 

 thick shells of Trichites and Astarte, as well as in 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 striae only occupying the central portion. 



Myopara is a name proposed as a genus by Lea in substitution of Stalaymium, 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 Crenella. It is of a more ovate outline than are the generality 



