192 "TERRA NOVA" EXPEDITION. 



connected at the apex. It is stated by Fischer and Oehlert [1891, p. 15] that the 

 septum commences at the summit of the valve. Their figures [1891, pi. i, figs. 2p and 

 2q] show no trace of posterior bifurcation ; nor does that of Davidson. 



With the above exception the interior details appear, from the figures, to lie- 

 similar in the two species ; the outer surface of the shell, too, in R. cornea is liable to 

 be undermined in the same peculiar manner as in the Antarctic species (compare PI. I, 

 fig. 9, with Fischer and Oehlert, 1891, pi. i, fig. 2u). 



The o-eocraphical rautre of /?. cornea is the Lusitauian Sub-region of the Atlantic 



O O i O ' 



Ocean, from the English Channel to the Soudanese Coast of Africa (Cape Bojador) ; the 

 range in depth is from 383 to 1,109 fathoms* ; the bottom temperature varies between 

 41 and 32 F. [F. and 0., 1891, p. 118]. 



The " Terra Nova " Rhynchonellid also presents a remarkable superficial resem- 

 blance to Fi'ieli'id /mill, Dull, from the N.W. coast of America. A close comparison, 

 however, of specimens with Ball's description [1895, pp. 713-716] and with two 

 examples of F. /mill recently received, shows structural differences which prove them 

 to be not only specifically, but generically, distinct. The most important difference 

 lies in the construction of the cardiualia. In F. halit. the latter are characterised by 

 the presence of a platform f consisting of two hinge-plates, excavate below, extending 

 outward from the inner sides of the crural bases and uniting in the median line over 

 the septum. This platform is solidly attached to the septum by means of a widened 

 surface,^ which supports part of each lamina as well as their line of junction. An 

 impressed mesial line is present on the upper surface of the platform, which is also 

 indented mesially and overhangs the septum in front. A cardinal process is sometimes 

 developed in old age. (Compare PI. I, figs. 5 and 8.) 



The outer surface of F. ludli is radially striated, but owing to the high polish the 

 strise are very indistinct. Owing to their fineness I have been unable to obtain a 

 satisfactory photograph for comparison with the " Terra Nova " species. 



F. ItaUi is evidently a non-plicate species subject to accidental distorsiou which 

 gives the shell a Bilobites appearance. It is also subject to attacks of boring organisms 

 similar to those of the Antarctic species. It ranges from latitude 47, off Grays Harbour, 

 Washington, to the Pacific Ocean, off San Diego, California; 559 to 984 fathoms; 

 bottom temperature, 38 to 39 F. 



Another species bearing some external likeness to the above forms is Plemithyns 

 rraitiuna, Ball, from the Gulf of Panama (1,175 fathoms ; bottom temperature, 36 8 F.), 

 ln.it, according to the description, the deltitlial lamellae are obsolete. The presence or 



* Davidsou [1887, p. 172] gives 57 i fathoms, off Cape St. Vincent. This is obviously a misprint for 

 577.1 fathoms (see F. A: O. 1891, p. 15). This error has rnisleil Schuchert, as he remarks on the occurrence 

 of this cold water species in the warm iraler off Cape St. Vincent [1911, p. 265]. 



t The term " spnndvlium " is used by Dall, but this is strictly applied to the spoon-shaped plate 

 frequently present in the ventral (or pedicle) valve of some Articulata (Pentamerus, etc.). 



:f The septum dues not bifurcate, as in the ' Terra Nova " species. 



