49 



of others where the growth has been interrupted, we 

 infer that a slig;ht sinus exists in the lip at the part where 

 the keel is formed in the two last whorls : the mouth 

 appears to be oval with an extremely short beak, or 

 rather, only a sinus in its lower angle, and an expanded 

 outer lip : the keel is placed about one-third down the 

 whorl, it is large and rises rather suddenly ; above it 

 the grooves before mentioned are sometimes decussated ; 

 a small portion of the dark epidermis is remaining upon 

 one specimen. 



I am indebted to Lady Burgoine and Miss Beminster 

 for the specimens figured of this rare Barton fossil ; I 

 consider it to be a fresh water shell, from some one of 

 the mixed beds above the London Clay of the Hamp- 

 shire coast. 



The decussated grooves had escaped observation 

 until I saw them strongly marked in a specimen kindly 

 lent me by the Viscount Fitzharris. The apex is com- 

 monly destroyed, and holes corroded in various parts of 

 its surface, by the action probably of some acid devel- 

 loped during the fermentation of vegetable matters, in 

 the marsh or at the bottom of the river where the animal 

 anciently lived, a circumstance that is not so likely to 

 arise in salt water, and has therefore been used to dis- 

 tinguish fresh water shells by. 



Figures 1 to 4 represent P. rigidus in different stages 

 of growth : figs. 5 and 6 are outlines of the mouth of 

 P. ater ; they shew the lip more enlarged than usual in 

 the genus, and not reflected or expanded, as in some 

 species. 



