148 CLASS I. TROPIOPODA. ORDER II. UNIMUSCULOSA. 



this genus that have little or no indication of the basal lobes (vide PI. CVII. 

 f. 2.). Our attention is again drawn to the hinge for a generic character : 

 instead of the parallel grooves which characterize the shell of Perna, the 

 hinge of Malleus consists of but a single central pit, protruding a little 

 inwards, the ligament being partly inserted between the dorsal area of the 

 valves, and partly in this central pit. The Mallei cannot well be con- 

 founded with any other genus : without reference to the presence or 

 absence of the lateral lobes, they differ both from the Pernce and the 

 Ostrecs, as we have already shown, in the structure of the hinge, and 

 from the latter especially in being provided with a byssus. 



The shell of Malleus may be described as being irregular, variously 

 distorted, mostly elongated, and generally lobed on each side at the base ; 

 the lobes are straight and the umbones divaricate. The hinge is destitute 

 of teeth, and has the ligament partly external in an area behind the valves, 

 and partly internal, within a small, central, triangular pit. The animal 

 is provided with a strong byssus, which passes out through a notch in the 

 lower valve behind the hinge. The muscular impression is compound. 



Examples. 



PI. CVII. Fig. 1. 



Malleus vulgaris, Lamarck, Anim. sans vert., new edit., vol. vii. p. 91. 



Enc. Meth., pi. 177. f. 12. 

 Ostrea malleus, Linnaeus. 



PI. CVII. Fig. 2. 



Malleus normalis, Lamarck, Anim. sans vert., new edit., vol. vii. p. 92. 

 Sowerby, Genera of Shells, No. 6. 



VULSELLA, Lamarck. 



Testa longitudinalis, fibro-laminaris, sequivalvis, subirregularis, umboni- 

 bus sequalibus. Galium cardinale in utraque valva prominulum, 



