42 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



as in JEtheria, sometimes wide apart and regular as the septa of an 

 Orthoceras\ the interspaces are occasionally empty, but are more 

 usually filled with calcareous spar. The whole inner stratum of shell 

 is frequently replaced by crystalline carbonate of lime. 



Figs. 2 & 3. — Sections q/* Hippurites cornu-vaccinum, Bronn. 

 Fig. 2. Fig. 3. 



Fig. 2. Upper lialf of a longitudinal section (^ nat. size), taken in the direction 

 d, b of Fig. 3, cutting only the base of the posterior tooth {f). 



Fig. 3. Transverse section of a larger specimen (f nat. size) at about the level d, h 

 of Fig. 2, cutting the point of the posterior apophysis (a')» and showing 

 the peculiar shell-texture deposited by the anterior adductor {a) : 

 I, m, n, duplicatures ; u, umbonal cavity of left valve ; r, of right valve ; 

 c, c', cartilage-pits ; t, f, teeth ; a, a, muscular apophyses ; d, outer 

 shell-layer ; e, inner sheU-layer. 



A longitudinal section shows the laminae of shell filling up the 

 interior nearly to the summit, leaving but a small space for the body 

 of the animal, now occupied by hard limestone. 



The upper valve serves as an operculum to close the aperture of 

 the lower valve, and does not thicken with age ; its outer stratum is 

 permeated by canals which radiate from the centre to the margin, 

 and give off small branches which appear as pores on the outer sur- 

 face. The inner layer is always metamorphic and crystalline ; it 

 gives off processes which penetrate to some depth the substance of 

 the lower valve. 



The interior of the lower valve has been figured and described by 

 Goldfuss and D'Orbigny. Goldfuss's figure (Petr. Germ. t. 164. 

 f. 1 c, p. 300) has been generally overlooked, perhaps because he 

 described it merely as an example of Radiolites agariciformis, 

 " wanting the upper layers." 



The British Museum has lately acquired a specimen precisely similar 

 (fig. 4). The inner layer exhibits an irregularly cellular structure, 

 which I have not met with in any other Hippurite, and to which its 

 excellent preservation is probably due. The cells are large, irregular, 



