BRACHIOPOD A JACKSON. 191 



perfect adult specimen lie was unalile to appreciate fully the remarkable likeness 

 of the two forms. Joubin's type specimen [11)01, pi. i. figs. 1-3] is ovoid in 

 form, with the valves regularlv rounded, aliont equally inflated, and possessing no 

 flattened part such as is present in A'. <-nrin'<i and in some of the " Terra Xo\a 

 examples. In Ji.t;>rn<'<i the two lateral angles are very much accentuated, which 

 gives the shell a more triangular appearance. One of the "Terra Nova " examples 

 (PL I, fig. 3 F) agrees exactly in size and form with Jouliin's type, and shows the same 

 features as those described, hut there are others which show that this species is subject 

 to considerable variation in outline and in rhe amount of flattening of the valves 

 (PI. I, figs. 3, G). 



In emphasizing the difference between It. <'nnn'n and A'. r<i<'r'il :<< .loubin remarks:-- 

 " Le contour de la commissure palleale est tres different dans les deux especes. 

 Anterieurement la valve superieure [ = dorsal] presente une chancrure niediane tres 

 nette, a laquelle correspond une saillie du bord de la valve inferieure [=ventral] qui 

 v 'ieut s'y engager. Dans R. crin><i c'est le contraire ; Tangle rentraut est sur la valve 

 iuferieure, et Tangle saillaut snr la valve superieure," which means that in his opinion 

 7?. I'uriii'ii is vent rail v uniplieate and R. n/rnr/fcir dorsallv uniplicate. The latter is a 

 characteristic feature of llhvnchonellids. 



I have been unable to obtain a specimen of A'. ^nrin;t in order to verify the above 

 statement, but the illustrations given by Davidson [1887, pi. -;">, figs. L'b and :!b] 

 would seem to indicate that this species is incipiently dorsallv uniplicate, though in his 

 description he distinctly states that the shell is "without either fold or sinus." The 

 figures given by Fischer and Oehlert [1891, pi. i.] show a lenticular condition with no 

 folding. In the text they state: "commissure palle'ale droite, parfois legerement 

 iucurvee au front." 



Another point of difference between the Antarctic species and /.'. cornea is the fact 

 that the longitudinal striae in the former are finer and somewhat more numerous than 

 in the latter. 



According to Fischer and ( >ehlert \n/>. c/'f.. p. 14], Jf. curnt'ii shows about sixtv radii 

 per cm. ; while, according to Joubiu [<>/>. cit., pp. 6 and 8] U. racovitzce possesses about 

 eleven radii per mm. (P. tji'rtur/H'i, on an average, nine per mm.) at the edge of the 

 valve. As stated previouslv, the " Terra Nova " examples show ei'jlit to ten (jiossiblv 

 eleven) radii per mm. They are clearly visible even over the umbonal region, \\hile 

 in I!, cornea, according to Joiibin, they are not distinguishable until further awav 

 from the beak. Possibly it will be found that A'. '///<>/ will exhibit similar differences 

 in the number of radii present <>n the shell. 



Regarding interior details it is difficult for a comparison to lie made in the absence 

 of a specimen of 11. <-<>rn<-<t. The descriptions of the cardinalia furnished bv Davidson 

 [1887] and Fischer and Oehlert [18!)1] are not sufficiently clear as to whether the 

 mesial septum of the dorsal valve is fused with the cardinalia, though from the 

 illustration given by Davidson [1887, pi. _'.">. li'j. 4j one might assume that it was 



