G. W. Lamplugh — The Yorkshire Boulder-clay. 65 



the umbo, even when the Boulder-clay shows no other sign of the 

 demolition of sand-beds. Until now I have found it difficult to under- 

 stand the preservation of sand in such a position, but this difficulty has 

 been cleared away by specimens which I have collected from the edges 

 of the deposit under consideration. These show how one valve lying 

 near the borders of a sandy seam may be driven forward into the 

 clay by a lateral squeeze or push, carrying with it the sand which 

 lay sheltered under its edges, while the other valve may remain 

 imbedded in the' sand. And when once a valve, is thus incorporated 

 with the clay, the sand under the umbo is effectively plugged in by 

 the clay, and it is easy to understand how it may remain there if 

 the shell remain intact, notwithstanding any further motion the mass 

 of the clay may undergo. 



Thus, I collected unbroken specimens of Astarle eompressa often 

 where the sand seam was so thin that both valves were nearly touching 

 the clay, in which, though the valves remained in contact, they were 

 twisted about so that the umbos did not coincide ; others in which one 

 valve had been pushed forward so as to touch the other at one point 

 only ; and others again in which the valves were quite separated ; 

 and in more than one case one valve with its contained sand had 

 been so far displaced as to lie in clay. 



It is somewhat curious to note that in the main seam the shells 

 had suffered most where the sand was thickest, and that unbroken 

 specimens were more common toward the extremities, where the 

 seam was tapering out, than in the centre. 



The following is a list of the species I have obtained from the 

 bed. All these species occur also both at Bridlington and at Dim- 

 lington. The list is small as compared with the list of over 100 

 species from the former place ; but this we might expect from the 

 very limited nature of the seam, which can only represent one small 

 portion of the sea-bottom, whereas at Bridlington not only were the 

 patches of larger size, but they also showed great diversity and had 

 evidently been brought together from different areas. 



List of Species found at South Sea Landing. 



c Pecten islandicus, Miiller. 



Mytilus modiolus ? L. 



Leda (limatula, Say?). 



Gardium groenlandicum, Ch. 

 c Cyprina islandica, L. 

 C Astarte borealis, Ch., and vars. 

 c eompressa, Mont. 



— : , var. striata. 



sulcata, Da Costa. 



Mi/a truncata, var. uddevallensis. 

 Saxicava norvegica, Speng'ler. 



rugosa, L. 



c Dentalium striolatum, Stimpson. 

 Ticrritella erosa, Couth. 

 Katica islandica, Gm. 

 Fusus despectus, L. 

 Admete viridula, Fabr. 

 Balanus [crenatus ?) Brag. 



Mya truncata, L. 



c indicates that the shell is plentiful. 



Almost everywhere on the headland the Basement Boulder-clay 

 contains a plentiful sprinkling of broken shells, which, along with 

 the sea-worn and Pholas-bored pebbles, are doubtless derived 

 from the complete destruction of beds such as these. But it is 

 worthy of note that, as in former instances, while in the clay TeUina 



DECADE III. VOL. VII. NO. II. 5 



