WOODWARD HIPPURITID^. 47 



ligamental inflection is seldom indicated externally ; but in those species 

 which M. D'Orbigny has distinguished as Bi-radiolites (fig. 19) 

 there is a peculiarly sculptured tract, or " cardinal area," on each 

 side the ligamental line ; and in R. Fleuriaui the upper valve is 

 rendered subspiral by a ligamental groove *. 



The interior of each valve is nearly symmetrical, but we may infer 

 from the structure of R. Fleuriaui, that the Radiolite, like the 

 Hippurite, was attached by the dextral valve ; an inference which 

 is confirmed by comparing R. polyconilites with Caprotina. The 

 outer wall or shell-layer of the Radiolite is unlike that of the Hip- 

 purite. It consists of prismatic-cellular structure like the shell of 

 the recent Pinna f ; and in all specimens from chalk strata the cells 

 are empty. As the shells of Pinna, Inoceramus, and Beletnnites in 

 the same formations are solid, we may conclude that this difference 

 was original and essential. 



Fig. 12. — Part of the rim of Radiolites Mortoni, Mantell, from the 

 Lower Chalk of Sussex. Traced from the original specimen in 

 the Museum of the School of Mines. 



I, the outer edge ; b, the inner edge ; v, v, the dichotomous impressions. 



fig. 1) it forms nearly all that usually remains of the upper valve, and all the 

 squamose ornaments of the lower valve. In the Hippurite it is seldom distin- 

 guishable ; but is quite distinct in Caprina and Caprinella. No such layer exists 

 in any of the Palliobranchiata. Mr. Sharpe has also pointed out that the long 

 prisms of the middle shell-layer are always parallel with the surface (and not 

 always perpendicular to the laminae of growth), like the tubes of Caprina and 

 Caprinella. In the Palliobranchiata the slender shell-prisms cross obliquely from 

 the inner to the outer surface, as shown by Dr. Carpenter. (See Davidson's Mo- 

 nograph of the Brachiopoda; Pal. Soc.)— Dec. 30, 1854, S. P.W. 



* Specimens of Radiolites Hceninghausii which have lost their inner shell-layer 

 present internally strong transverse furrows, or lines of growth, and a prominent 

 ligamental ridge. The interior of the upper valve, when in this condition, shows 

 that the umbo of the young shell was marginal, as represented in PI. V. fig. 3. I 

 cannot distinguish R. acutus, D'Orb., from the young of R. Hceninghausii. — 

 Dec. 30, 1854, S. P. W. 



t The cells of Radiolites from the English Chalk are twenty-five times as large 

 as those of the recent Pinna ; and those of Pinna are 250 times as large as the 

 cells of Pandora (Carpenter). 



