44 THE VOYAGE OF H.M.S. CHALLENGER. 



is the oldest, and the one it is desirable should be retained for the species. It is very 

 variable in shape. The shell in some specimens, np to nearly half the size of the largest, 

 is almost entirely smooth, while other examples are ribbed almost up to the beak and 

 umbo. Some are stinted in growth, and with their beak more than usually incurved 

 {Terebratula dentata and Terebratula recurva). The ribs vary likewise to a considerable 

 extent in number, size, shape, and direction. 



Waldheimia wyvillii, n. sp. (PL III. fig. 13, a, h). 



Shell ovate or longitudinally oval, very thin, semitransparent, light brownish-yeUow, 

 smooth, marked at intervals by concentric lines of growth. Dorsal valve moderately con- 

 vex longitudinally, slightly flattened along the middle. Ventral valve deeper or more 

 convex than the dorsal one, withoiit sinus. Beak incurved, truncated by an incomplete 

 foramen margined laterally by small deltidial plates. In the interior of the dorsal valve 

 the loop is long and simple, the lateral branches extending to a little beyond two-thirds 

 of its length before becoming reflected. Length 19, width 14, depth 10 lines. 



Habitat. — Only one incomplete example of this species was dredged by the Chal- 

 lenger Expedition, off Valparaiso, at Station 299, on December 14, 1875, in lat. 

 33° 31' S., long. 74° 43' W., at a depth of 2160 fathoms. Bottom temperature, 1°-1 C. 

 Sea bottom, grey ooze. Terebratida wyvillii and Discina atlantica were obtained at 

 the same time. 



Observations. — In external shape this species approaches Waldheimia cranium, which 

 is, however, a thicker and more convex shell. The extremely delicate shell of Waldheimia 

 ivyvillii is very remarkable, and reminds us of Waldheimia tenera, Jeffreys, but from 

 which it seems to differ in size and some other particulars. The fact that several 

 of the species obtained at such great depths, such as Terebratula ivyvillii and Discina 

 atlantica, possess such exceedingly thin and delicate glass-like shells is certainly worthy 

 of notice. 



Terehratella, D'Orbigny. 



Terebratella dorsata, Gmel., sp. (PL IV. fig. 4). 



Anomia striata magellanica, Chem., Concli. Cab., vol. viii. p. 101, pi. Ixxvii. figs. 710, 711. 



Anomia dorsata, Gmel., s. n., 3348, 1788. 



Terehratula dorsata. Lam., Anim. sans Vert., vol. vi. p. 246, 1819. 



Terebratula soioerhyi, P. P. King, Zool. Journ., vol. v. p. 339, 1835. 



Delthyris dorsata, Menke, Syst. Syn. Mollusc. Gen., 2d edit, p. 96, 1830. 



Terebratula bilobata et pectinata., Blainville, Teste Eev. Journ. de Conch., p. 127, 1861. 



Terebratula magellanica, L. Eeeve, Concli. Icon., pi. v. fig. 21. 



Terehratella doi-sata, Davidson, Dall, and of several malaoologists. 



Shell somewhat transversely oval, wider than long ; valves moderately convex, light 



