UPPER, OR FLINTY CHALK. 215 



This shell is very gibbous, the convexity of the united valves being 

 equal to its length. The valves are regular, and gradually increase in 

 width from the hinge to the front ; there are about twenty prominent 

 ridges, or costae, on each, which diverge from the posterior slope over 

 the convexity of the shell, and terminate with a gentle curve on the 

 border of the hinge. The posterior side is flat, the anterior acute, form- 

 ing an angle with the hinge ; the latter is transverse, its plane cor- 

 responding with the transverse diameter of the valves. The front is 

 rounded, the margin very entire; the beaks small and reflexed. 



The present species may be distinguished from the preceding by the 

 number and strength of the costas; the flat, truncated, posterior slope; 

 and more particularly, by the hinge side, which is not expanded as in I. 

 Lamarckii. 



I have named this elegant shell in honour of my friend M. Brongniart, 

 the able colleague of Baron Cuvier, author of " Geographie Miner alogique 

 des Environs de Paris" &c. 



Tab. xxvii. fig. 8, an excellent specimen in which the characters of the 

 species are well defined ; the beaks are broken off. This figure is reduced 

 to one-half the size of the original. 



Tab. xxviii. fig. S, an imperfect example of a variety of this species. 



Localities. Upper chalk, near Lewes, and Brighton. 



86. Inoceramus mytilloides. Tab. xxviii. fig. 2. 



Depressed, elongated, with numerous concentric striae, and a few dis- 

 tant ridges; posterior slope inflated, its margin plicated; anterior side de- 

 pressed, expanded ; beaks acuminated ; hinge very obhque. 



The shell is most convex near the beaks, and gradually becomes flatter 

 and more expanded towards the margin. The beaks are produced, and 

 terminate in a sharp point, over the commencement of the hinge; the 

 plane of the latter is placed at an acute angle with the longitudinal dia- 

 meter of the valve. 



The striae are numerous, and form a few irregular ridges that terminate 

 in folds, on the margin of the posterior slope. Fragments of this species 



