190 "TERRA NOVA" EXPEDITION. 



bifurcation of u mesial septum (PI. I, fig. 5). Anterior to this bifurcation the septum 

 is sharp-ed^ed and well-defined, and extends forward to about a third the length of 

 the valve, separating the four distinct scars of the adductor muscles, of which the two 

 anterior are the largest. In some cases these muscle-scars extend slightly in front of 

 the end of the septum. There is no obvious cardinal process, the diductor muscles 

 being attached to the posterior parts of the crural bases and socket-ridges. In neither 

 valve is there any trace of the furrows for the pallia! sinuses. 



The foregoing general description is applicable to the majority of the "Terra 

 Nova " specimens. A few others show certain deviations. In young shells the form 

 is more regularly ovate (much as in Macandrevia cranium), and the folding is 

 incipient (PI. I, tig. G). An old thick-shelled example from Station 194 shows 

 considerable calcification in the nmboiial cavity of the ventral valve, with nearly 

 complete obsolescence of the dental plates and pedicle-collar.* In the dorsal valve 

 of this specimen the cardinalia are similarly thickened, and the posterior inner sides 

 of the crural bases almost meet in the median line over the septum, leaving, however, 

 a tiny cavity below the apex. 



The valves of several of the shells are pierced with small circular holes, doubtless 

 owing to attacks by carnivorous gastropods ; others are partly overgrown by Polyzoa. 

 Many present a curious feature when viewed under the microscope, owing to the fact 

 that the external surface of the shell is undermined by a network of fine strings 

 connected with enlargements which are not uniform in shape (PI. I, fig. 9). The nature 

 of the organisms which form these burrows is not certain. 



In 1901, .loubin described, as two new species, some thin-shelled, radially-striated 

 Rhynchonellids which were dredged by the " Belgica " Expedition in 192 to 275 

 fathoms in the Western Antarctic. The first species, to which he gave the name of 

 Rhynchonella racovitzce, was founded upon a single specimen containing the animal, 

 and a fragment of a ventral valve showing interior details. The second species 

 (JR. i/i.'i-lnc/tci) was based upon two small examples showing obvious juvenile features. 

 In my opinion it possesses no definite characters which separate it from ./?. racovitzce, 

 and I feel convinced that it cannot be regarded as more than a young stage of that 

 species. 



The types of the above are in the Brussels Museum, and are, therefore, not 

 available for study ; but from the excellent descriptions and figures given by Joubin 

 [1901] it is evident that the " Terra Nova " Rhynchonellid is identical with R. ntcucit-a'. 

 The discovery of this species in the Ross Sea area thus extends the range very 

 considerably. 



The resemblance of this species to Rhynchonella cm-iirx (Fischer MS.), Davidson, is 

 very striking. This fact was noted by Joubiu, lint as he was only in possession of one 



* In old adult shells of HemitJiyris pxittacea the pedicle-collar is sometimes fused to the floor of the 

 umbonal cavity, and the dental plates tend to become obsolete through excessive calcification. 



