POST-TERTIARY FOSSILIFEROUS DEPOSITS. 59 



Trochus helicinus is remarkably prevalent, and in no one of the glacial beds we have 

 examined has it been found so abundantly as in this locality. 



Saxicava rugosa is next in abundance, and the var. arcUca appears, although not of 

 marked size, few specimens reaching an inch. Portions of the epidermis are preserved. 

 A few fragments of Mi/a truncata are met with covered with fresh epidermis, and, 

 therefore, evidently washed from a near part of the bed where they must be in situ. 



Littorina litorea, L. obitcsata, and L. limata (which are all very scarce, especially 

 L. limata) are imperfect and water-worn. 



There is one curious circumstance characteristic of these local groupings, in which 

 the facts connected with the fossils coincide with those connected with recent deposits, 

 and which it is important for the geologist to consider when collecting pala3ontological 

 evidence regarding the ages of the beds. Almost all the deposits have their peculiarly 

 prevalent species. A rare species in one bed may be abundant in another, belonging to 

 precisely the same age. 



In the East Tarbert bed Pseiidocythere caudata, of Sars, is moderately common. 

 It is recorded as rare between tide marks in the Channel Islands ; rare in England, 

 Scotland, Ireland, and Norway in deep water ; and it is also rare in the glacial clays. 



Cythere emarginata (Sars), a still rarer species, is met with in the same bed ; male 

 and female specimens being moderately common. 



This species has been found living only in one or two localities in the British seas, 

 and there very sparingly. 



Height above the sea twelve to fifteen feet. 



The following Ostracoda were found 



'a 



Cythere pellucida, Baird. 



— castanea, G. O. Sars. 



— viridis, Miiller. 



— lutea, Midler. 



— villosa (G. O. Sars). 



— angulata (G. O. Sars). 



— emarginata (G. O. Sars). 



— concinna, Jones. 



— tuberculata (G. O. Sars). 

 Cytheridea papillosa. Bosquet. 



— punctillata, Brady. 

 Cytherura nigrescens (Baird). 



— undata, G. O. Sars. 



