TEREDO. 177 



sion, occupying at least one-half of that side, and raised above 

 the rest of the shell : beaks much incurved, situate near the 

 anterior end, at about one -third the length of the dorsal line ; 

 umbones prominent: hinge-line very irregular: hinge-plate 

 very broad and extremely solid, folded over the anterior dorsal 

 area, which represents a thickened sinuosity; it is deeply 

 notched on the other side, in consequence of which the auricle 

 rises more abruptly; the centre is furnished with a large 

 callous protuberance or knob, as well as with a short peg-like 

 tooth or prong, which is stronger and more conspicuous in the 

 right than in the left valve: apophyses rather narrow and regu- 

 lar, not much curved, but occasionally twisted ; inside glossy, 

 furnished in front with a rather large and solid pear-shaped 

 excrescence ; the auricle is separated by a slight and indistinct 

 rib, but there is no shelf or ledge such as is observable in all 

 the other species before described : muscular scars distinct ; 

 the muscles themselves adhere very closely, and can be easily 

 seen in living specimens; anterior narrow and placed ob- 

 liquely across the centre of the hinge-plate ; posterior broad 

 and large, occupying about one-half of the auricle : pallets 

 large and leaf-like ; blade oval, squarish in front, slightly 

 convex outside and concave inside, covered with a glossy white 

 epidermis; the outside front is wedge-like and partly ex- 

 cavated by a semicircular impression (exposing the laminated 

 structure of the blade), which extends inwards over one-third 

 or more of the blade ; stalk short, stake-like, more solid than 

 the blade ; it is continued on both sides far into the blade, 

 and on the under side may be traced the whole way from one 

 end to the other, like a midrib; the upper surface of the 

 blade near the insertion of the stalk is sharply excavated on 

 each side, but not to any great distance : sheath usually thin, 

 except at the neck, which is lined with imbricated plates, and 

 these latter are crossed by a sharp siphonal ridge on either 

 side. Valves, L. 0-4, B. 0-4 ; pallets, L. 0-4, B. 0-15 ; 

 sheath, L. 3-6, B. 0-45. 



Yar. 1. excisa. Shell similar to the stunted variety of each 

 of the foregoing species. 



Var. 2. striatior. Shell more convex and not so solid ; an- 

 terior area larger, and more closely and finely striated ; hinge 

 callosity not so prominent. 



Yar. 3. mionota. Shell smaller, with the auricle less de- 

 veloped and not reaching so far down ; pallets shorter, having 

 the semicircular part in front more deeply excavated. 



i 5 



