. Annals of the South African Museum = Annale van die Suid-Afrikaanse Museum. Natural history. CONTRIBUTIONS TO THE KNOWLEDGE OF MARINE MOLLUSCA 457 Crassatella natalensis Fig. 19 b Subquadrangular, moderately convex, obliquely truncate behind, rounded in front, umbonal ridge distinct, postero-dorsal margin straight, posterior margin straight, sometimes very slightly concave, ventral margin not very convex; faint traces here and there of very fine radial striae on some valves (sed ?); concentric lirae c. 10 at 2 mm. alt., 18-20 at 5 mm., 29-35 at x4 nun., rounded, about as wide as t
. Annals of the South African Museum = Annale van die Suid-Afrikaanse Museum. Natural history. CONTRIBUTIONS TO THE KNOWLEDGE OF MARINE MOLLUSCA 457 Crassatella natalensis Fig. 19 b Subquadrangular, moderately convex, obliquely truncate behind, rounded in front, umbonal ridge distinct, postero-dorsal margin straight, posterior margin straight, sometimes very slightly concave, ventral margin not very convex; faint traces here and there of very fine radial striae on some valves (sed ?); concentric lirae c. 10 at 2 mm. alt., 18-20 at 5 mm., 29-35 at x4 nun., rounded, about as wide as the grooves but sometimes narrower, up to alt. 2 mm. evenly rounded from anterior to posterior margin, thereafter rather sharply bent at right angles at the umbonal ridge. Adductor muscle scars impressed, margin internally crenulate. The freshest valves with faint traces of orange radial, somewhat discontinuous Fig. 19. a, Crassatella burnupi Lamy. copy after Lamy. b, C. natalensis , with profile, c, C. pilual , with profile, and profiles of lirae at different stages. Length 20, alt. 14, thickness (one valve) 4-5 mm. Off O'Neil Peak (Zululand), 90 fathoms, 11 valves; off Umhloti River (Natal), 40 fathoms, one valve; off Cape Natal (Durban), 54 fathoms, 4 valves andonejuv. (Types) (S. Afr. Mus. Nos. A9509 (Types), A9510, A9511. coll.). Remarks. If these smaller valves should prove to be burnupi, the 'cause accidentale' (Lamy) of only one worn valve of burnupi having been washed ashore might perhaps be the deep water habitat at which the present valves were dredged. The feature distinguishing these valves from other South African species is the right-angled bend of the lirae at the umbonal ridge (more marked than in tenuis). This appears also in burnupi, but in the latter the shape of the area encompassed by the umbonal ridge, the anterior and the ventral margin approximates far more to an equilateral triangle than in the present valves. The course of the
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectnaturalhistory, booky
