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



Observations. — This is one of the most interesting species of deep-sea Brachiopoda dredged 

 during the Challenger Expedition. The shell is of such extreme thinness that it is almost 

 transparent ; indeed, the valves when separated are really so, and the muscular impressions 

 may be seen through its transparency. It is also exceedingly brittle. I separated the 

 valves of a specimen in order to be able to study its animal and loop, the latter, which 

 I was very much surprised to find short, is in every respect similar to that of Terebratula 

 proper, to which genus the species must be referred, notwithstanding its outward 

 Walclheimia-Yike appearance. It bears much resemblance to several species of the last- 

 named genus occurring in the Jurassic and Cretaceous formations and especially so to 

 Terebratula boneti, Zeuschner, from the Kimmeridge of Switzerland, and from which 

 some of the Challenger specimens are scarcely distinguishable, either by size or shape. 



Prior to making a complete examination of its interior characters I had provisionally 

 referred it to Waldheimia. We find but few recent species with such a thin shell, but 

 among these last may be named the far-spread Discina atlantica. King, Atretia 

 gnomon, JeflFreys, Waldheimia tenera, Jeffreys, Rhynchonella lucida, Gould, and one or 

 two others. When quite young and up to 4 or 5 mm. in length, the shell shows no 

 mesial depression, this begins at a more advanced age. I have much pleasure in naming 

 this species after Sir Wyville Thomson, F.E.S., the distinguished director of the civilian 

 staff on board the Challenger. 



Terebratula ciihensis, Pourtales (PI. II. figs. 10, 11). 



Terebratula cnbensis, Pourtales, Bull. Mus. Comp. Zool., vol. i., No. 7, p. 109, 1867. 

 Terebratula eubensis, Dall, Eeport on the Brachiopoda obtained by the United States Coast Survey 

 Expedition, Bull. Mus. Comp. Zool., vol. iii., pi. i. fig. 2, 1871. 



Shell globose, somewhat trigonal or obscurely pentagonal, longer than wide, smooth, 

 marked at intervals by concentric lines of growth, nearly white ; broadest anteriorly, 

 tapering posteriorly. Dorsal valve convex, without fold or sinus, front-line nearly straight 

 or slightly curved ; margin of valves laterally flexuous ; ventral valve somewhat deeper 

 than the opposite one, longitudinally and broadly flattened, the sides of the flattened 

 portion sloping away rapidly and rather abruptly on either side, giving the valve a some- 

 what subquadrangular aspect. Beak moderately incurved and truncated by a circular 

 foramen separated from the hinge-line by a narrow deltidium. Loops, short simple. 

 Length 28, width 22, depth 18 mm. 



Habitat. — Two examples were dredged off Ascension Island on April 3, 1876, at Station 

 334. Depth, 420 fathoms. Hard ground. 



Observations. — The two living specimens dredged by the Challenger Expedition agreed 

 in every respect with those obtained by M. de Pourtales, in from 100 to 300 fathoms off the 

 Florida Reefs, or off Havana in 270 fathoms. The shell was briefly described in 1 867 by M. 

 de Pourtales, and subsequently in 1871 with minute details by Mr W. H. Dall. The loop 



