i9i8. Phillips & Stelfox. — Range of Pisidiimi hihemicum. 37 



MENT Pit — short, broad and distinct. C. 3^1ong, straight or but 

 sHghtly curved. C 3 (the posterior portion of) — usually slightly 

 thickened. C. 2 — very long and straight. P. III. — parallel to p. I. 

 (PI. I., tig. 2b). 



P. nitidum Jenyns : — Shell — slightly inequilateral. Umbones — broad, 

 not very prominent. Fry — oval. Striation — regular, wide- 

 Siphon — funnel-shaped, with crenate margin (PI. I., fig. la.) 

 Ligament Pit — short, but not so broad nor so distinct 

 as in P. hiberniciim. C 3— short, usually distinctly curved or 

 crescent-shaped. C. 3 (the posterior portion of) — sometimes 

 thickened and even faintly bifurcate. C. 2 — short and straight 

 (usually only J or | the length of that of the associated P. hibernicuni) . 

 P. III.— parallel to p. I. (PL I., fig. ib). 



P. milium Held : — Shell — slightly or considerably inequilateral. Um- 

 bones — broad. Fry — oval. Striation — irregular, very strong. 

 Siphon — very narrow and very long. Ligament Pit— long, narrow 

 and indistinct. C. 3 — long and fairly straight. C. 3 (the posterior 

 portion of) — slightlv thickened. C 2— long. P. III. — parallel to 



P. obtusale (Lam.) Jenyns : — Shell — almost always inequilateral. 

 Umbones — broad. Fry — usually oval. Striation — irregular, very 

 close, but distinct. Siphon — narrow, margin simple (Plate I., fig. 

 3a). Ligament Pit — short, but rather indistinct. C. 3 — short and 

 curved. C 3 (the posterior portion of) — thickened and " hooked." 

 C. 2 — very short (upper portion sometimes bent outwards ; see B. B. 

 Woodward, Cat., p. 121). P. III. — coalescing with p. I. (or only 

 tending to in peaty- water forms), and forming a " pseudo-callus " 

 (PL I., fig. 3b). 



P. personatum Malm. : — Shell — equilateral. Umbones — broad. Fry — 

 very oval. Striation — almost imperceptible. Siphon — broad 



and short (PL I., fig. 4a). Ligament Pit — long and broad 

 as in P. casertanimi. C. 3 — short and curved. C. 3 (the posterior 

 portion of) — thickened and " hooked." C 2 — long and usually 

 curved. P. III. — parallel to p. I. (but above p. III. is a separate 

 callosity : the callus of B. B. Woodward, see Cat., p. 55) (PI. I., 

 fig. 4b). 



From P. lilljehorgi of equal size P. hihemicum may be at 

 once distinguished by its short and broad Hgament-pit, the 

 pit of P. lilljehorgi being longer and narrower than that of 

 any other species we have examined. Young shells oi the 

 two species in which the ligament pit is not easily dis- 

 cernible may, in cases, be extremely difficult to separate. 

 Neither the globular form nor the twisted lateral teeth of 

 the hinge referred to by Mr, Woodward as the chief diag- 



A 3 



