56 MOLLUSCA FROM THE CRAG. 
Generic Character. Shell equivalve, inequilateral, irregularly and roundedly trape- 
zoidal ; valves sometimes smooth or slightly sulcated concentrically ; sometimes entirely 
covered with radiating striz, sometimes the central portions smooth with the lateral 
extremities striated; anterior side very short; umbo subterminal; hinge margin 
linear, generally smooth, occasionally crenulated or denticulated ; ligament internal ; 
impressions of the adductor muscles different in form and unequal in size; anterior 
one small and elongato-ovate ; posterior one large and subcircular; impression of 
the mantle entire ; shell slightly gaping for the passage of a byssus. 
Animal of the form of the shell, and the margins of the mantle without a fringe: 
an elongated and cylindrical foot, with a gland at its base for the formation of a 
byssus. 
This genus has by some conchologists been united with the preceding one, in con- 
quence of some similarities between the animals as well as the shells. In this the 
animal differs in having a simple margin to its mantle, as well as a marked peculiarity 
in the branchial region, these characters are as distinct as are generally employed for 
the separation of genera, and in the shells the anterior side is always more or less 
pushed beyond the umbo, so as to give it a Jess triangular or a more trapezoidal form 
than in Mytilus. The aberrant species will, it is true, bear a close generic resemblance, 
and the line of demarcation is difficult to define, but the same may be said of most 
proximate genera. Some modern conchologists have constituted a new genus for those 
species which are externally ornamented or striated, a character here considered 
insufficient for generic distinction, more especially as in well-determined species of the 
preceding genus the shell is sometimes smooth, while in others it is covered with deep 
and strongly marked lines of radiating strie. 
It is doubtful also whether a line of crenulations upon the dorsal edge of the shell 
is a character sufficient alone for generic distinction. The genera Crenel/a and 
Modiolarca have therefore been included in the synonyma. Animals of this genus 
generally spin a byssus, by which they are attached, and the shell gapes a little at 
the anterior part of the ventral margin for its passage; several species in the recent 
state supply this material so largely, as to wholly invest the shell in a kind of nest: 
while others closely resembling this genus are capable of forming a habitation in the 
interior of calcareous rocks. (Mytilus lithophagus, Linn.) 
This is truly a Marine genus, and found at various depths, and is known among 
the oldest of the Secondary Rocks ; but it is rather sparingly distributed throughout 
the Tertiaries. 
