74 PALEONTOLOGY. 



the attachment of the adductor muscles, which become con- 

 tinually more undercut in the course of their growth. In 

 Hippurites the anterior muscular plate projects horizontally, 

 the posterior vertically, like a third tooth, for which it has 

 been mistaken. In this genus there are two longitudinal 

 inflections of the outer shell-wall beside the ligamental 

 furrow, one corresponding to the posterior muscular plate, the 

 other (or third) apparently a siphonal inflection like that in 

 Trigonia and Leda (fig. 19, 4). 



The structure and affinities of Hippurite shells are ably 

 treated of by S. P. Woodward in the " Quarterly Journal of 

 the Geological Society," Feb. 1855, pp. 40-60. Some had 

 deemed them to be cephalopodous, some brachiopodous, and 

 others annellidous : but Cuvier's view of their lamellibranchi- 

 ate nature is established, and the position assigned to the 

 Hippurites by Quenstedt and Woodward, between the Chama- 

 cece and Gardiadce, is now accepted. 



The cockle-shells (Cardiadce), as they have a world-wide 

 distribution now, had a corresponding range in time, and are 

 found in all strata from the Silurian upwards. The com- 

 monest fossil tribe of Gardium is ribbed concentrically on the 

 sides, and radiately on the posterior slope, a style of ornament 

 almost unique amongst the 200 recent species. The Caspian 

 cockles, distinguished by a sinus in the pallial line, appear to 

 have inhabited the Aralo-Caspian region almost from the 

 middle tertiary period; the hinge-teeth are reduced to one 

 (Monodacna) or two (Didacna) in each valve, and are some- 

 times quite wanting even in the young shell (Adacna, Eichw.) 

 Liihocardium aviculare (fig. 20, 7) is a characteristic shell of 

 the Paris basin, and appears to have spun a byssus, like the 

 fry of some recent cockles; it also resembles the oriental 

 Tridacna, of which a species is found in the miocene of 

 Poland. The genus GonocardAum (fig. 17, 8) of the upper 

 Silurian and carboniferous systems is remarkable for the 



