MONTACUTA. 211 



rior side produced considerably beyond the shell, and forming 

 a kind of frill, which becomes gradually smaller and more 

 even as it passes along the front of the shell towards the pos- 

 terior side ; it is fringed with very delicate, rather short, and 

 blunt filaments, which extend completely round the edges of 

 the valves, with the exception of a small space at the umbones : 

 tube consisting of a small inconspicuous excretory orifice : foot 

 very large and muscular, slightly bent in the middle, tapering 

 to a blunt point in front and abruptly truncate behind ; its 

 base or sole is somewhat wavy and grooved through its entire 

 length. 



Shell oblong, convex, thin, rather opaque, glossy : sculp- 

 ture, irregular lines of growth and occasionally a few exceed- 

 ingly slight longitudinal scratches : colour greyish -white : 

 epidermis as in the last species, but it is usually obscured by 

 a thick ferruginous crust: margins somewhat truncate but 

 rounded at the smaller or posterior end, thence obliquely 

 slanting and slightly curved in front, expanding and rounded 

 at the anterior end, and very gently sloping behind from 

 the beak to that side, with a short ledge on the other side 

 which forms an obtuse angle at the posterior extremity : beaks 

 small, blunt, and rather tumid, not very prominent, and nearly 

 straight; they are placed at about one-third the distance 

 from the posterior end : hinge-line almost straight, occupying 

 about one -third of the circumference : cartilage large and 

 solid, light yellowish- brown or horncolour, obliquely twisted 

 and clasping the hinge-plate on each side, lying close to the 

 beaks at the posterior side ; the pit or groove containing it 

 slants abruptly and obliquely down towards the posterior side, 

 and has its walls and base much thickened: hinge-plate nar- 

 row but thick : teeth, in the right valve one on the anterior 

 side, which runs from the cartilage at nearly a right angle to 

 the hinge-line and so far resembles a cardinal tooth, but it 

 then takes a sharp twist in the direction of the hinge -line, 

 where it becomes laminar and is gradually attenuated to a 

 sharp point ; the other tooth in the same valve on the posterior 

 side is shorter, triangular and pointed, placed on a lower level 

 and parallel with the posterior slope ; in the left valve the 

 tooth on the anterior side is pointed near its commencement 

 and forms a rather long laminar ridge in a parallel line with 

 the hinge, the other tooth being of the same shape as the corre- 

 sponding one in the right valve and taking a similar direction : 

 inside partly nacreous but mostly of a dull hue, with a plain 



