FAMILY 2. MYTILACEA. 135 



Family 2. MYTILACEA. 



Testa sublonga, regularis, aequivalvis, plerumque laevis. Cardo edentu- 

 lus, ligamento valido, lineari. Impressio muscularis composita. 

 Animal bysso serico filatim affixum. 



The Mytilacea have little affinity with the Tridacnacea, except in being 

 provided with a byssus ; and in this family that remarkable character, 

 which is not found in all the Unimuscular Tropiopoda, is very fully 

 developed. The byssus or beard consists of a number of filamentous or 

 silken threads, by which the animal attaches itself to rocks or other ma- 

 rine bodies, and by the assistance of the foot it has the power of either 

 fixing or displacing it. We may often observe the common muscles 

 adhering to each other by their byssi ; and in the Pinnae, which often 

 grow to an enormous size, the byssus has all the appearance of a large 

 bunch of silk. 



The shell of the Mytilacea may be described as being rather of an 

 elongate form, regular, equivalve, and generally smooth. The hinge is 

 entirely destitute of teeth, consisting merely of a strong marginal liga- 

 ment. The muscular impression is compound, bearing the marks of one 

 or more small accessory cartilages, probably destined to assist the muscle 

 in counteracting the strong expanding power of the hinge ligament. 



The family of the Mytilacea are divided into the four following 



llthodomus. mytilus. 



Modiola. Pinna. 



LlTHODOMUS, Cuvier. 



Testa transversa, cylindraceo-oblonga, sequivalvis, epidermide fusca in- 

 duta ; extremitatibus rotundatis, antica breviore ; umbonibus vix 



