PSAMMOBIA. 399 



of growth are conspicuous ; the whole surface is marked by 

 slight and irregular longitudinal or radiating lines, as well as 

 by close-set microscopical striae in the same direction, but 

 these stria? are generally effaced ; in young shells the posterior 

 slope is sculptured by radiating ribs as in P. Ferroensis, but 

 much less distinctly : colour beneath the epidermis yellowish- 

 white, variegated by bright rays of a purplish-brown or lilac 

 of different widths ; whether the rays are present or not, the 

 shell is frequently marbled, speckled, or spotted with white, and 

 lineated with short dusky streaks ; now and then specimens 

 are found having the umbones of a uniform purple hue : epider- 

 mis rather thick and persistent, light yellowish-brown in the 

 young, and of a deeper or olive -brown in the adult : margins 

 not much rounded in front, obliquely curved towards the an- 

 terior side, where they are semioval and wedge-shaped, slightly 

 indented or flexuous on the ventral side of the lower or poste- 

 rior angle, abruptly and somewhat obliquely truncate but not 

 sharply angular on the last-mentioned side, which is indis- 

 tinctly denned by an obscure ridge extending from the beak to 

 the lower angle ; dorsal margin nearly straight or but little 

 inclined, and ending in an obtuse angle which forms the upper 

 side of the posterior margin : beaks as in the last species : 

 ligament short, extremely prominent, annular and of a closely 

 fibrous texture, funnel-shaped, with the smaller end proceed- 

 ing from between the beaks and widening outwards ; the large 

 end is closed and held down to the dorsal edge by a membra- 

 no as extension, which in the young is of a yellowish colour, 

 the body of the ligament being horncolour; the valves are 

 separated in that part as in the other species : hinge-line ob- 

 tusely angular or gently curved : hinge-plate rather thick, 

 broader on the posterior side, abruptly truncate at the outer 

 end of the ligament : teeth as in P. Ferroensis : inside thick- 

 ened, porcellanous, highly polished and slightly iridescent, yel- 

 low or lemoncolour tinged with purple near the beak, some- 

 times of a uniform lilac, marked with faint longitudinal striae 

 towards the front margins, and microscopically wrinkled like 

 shagreen : pallial and muscular scars as in the other species. 

 L. 1-25. B. 2-25. 



Yar. 1. lactea. Shell milk-white ; all the teeth more erect 

 and recurved, and the smallest in the left valve triangular and 

 pointed instead of laminar or ridge-like. 



\ r ar. 2. livida. Shell more oval, or longer in proportion to 

 its breadth, of a uniform fawncolour, but shaded, and marked 



