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 striae, 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 less 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 striae. 



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 Crenella 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. 



