MONOMYARIA. 131 



coloured, with dark reddish-brown blotches; inside perla- 

 ceous and iridescent. One inch and a fourth long. Inhabits 

 the Devonshire coast. 



Genus 24.— MALLEUS.— Lamarck. 



Generic Character. — Subequivalve, rugged, distorted, 

 frequently elongated; sublobate at the base; beaks small, 

 diverging; hinge without teeth, with an elongated conical 

 furrow under the beaks ; ligament subexternal, short, placed 

 in the sloping area, at the base of the valves. 



Approximating in form to Perna, but very different in tlie structure 

 of its hinge, and in this respect more nearly resembling Avicula, 

 though it cannot be mistaken for it, both being without the sulcated 

 teeth or joints at the hinge ; but the conical cavity, situated under the 

 beaks of the Malleus, and crossing the angle of the slope of the liga- 

 ment, at once distinguishes it from Avicula. 



Malleus albus. — The White Malleus. Plate XV. 

 fig. 12. Trilobate; lateral lobes of the base prolonged, 

 sinus none, or not distinct from the pit for the ligament • 

 white, with transverse undulations. Four inches long. 

 Inhabits the Australian seas. 



G nus 25.— PERNA.— ZamarcA. 



Generic Character Subequivalve, flattened, slightly 



distorted, of a lamellar texture ; hinge linear, marginal, and 

 many toothed; these are furrow-like, parallel, and trans- 

 verse, not inserted in the opposite furrows, but with the 

 ligament inserted between them ; sinus for the byssus some- 

 what gaping, and placed under the extremity of the hinge. 



The substance of the shell, although solid, is formed of flaky por- 

 tions, not adhering closely to each other, and giving it a foUac eous 

 appearance. 



Perna Ephippium The Saddle Perna. Plate XV. 



fig. 13. Compressed, suborbicular ; hind side produced, the 

 margin very acute ; purplish-brown on the outside, and pearly 

 within. Four inches long. Inhabits the Indian seas. 



