President's Address. 23 



Slimon and Page. We may now resume this branch of our 

 subject. 



The addition of a considerable suite of Mollusca by the 

 Geological Survey after this period was first announced by 

 Professor Geikie, in his valuable paper "On the Order of 

 Succession amongst the Silurian Eocks of Scotland." * 



A small elongated Lingida is common at Lesmahagow 

 associated with the remains of large crustaceans, and before 

 referred to under the name of L. cooiiea. The researches of 

 Mr Davidson f have placed it beyond doubt that the form 

 found at Lesmahagow should more properly be referred to 

 another species, the L. minima (Sby.). Lingida cornea is the 

 characteristic species of the " passage beds " of Murchison, or 

 those strata intermediate between the uppermost Ludlow 

 beds and the base of the succeeding Old Eed sandstone in the 

 west of England. On the other hand, L. minima is typical 

 of the Downton sandstone, or highest zone of the Upper 

 Silurian of the same region. It will be thus seen that the 

 23roper identification of this little shell had a very interesting 

 bearing on the exact position held by the Lesmahagow series 

 in the geological scale, when compared with other districts. 



The " grey and reddish shales and sandstones " of Lesma- 

 hagow, known as the "Lingida and Troclms beds," J: are very 

 interesting to the conchologist from the innumerable numbers 

 in which the little shell called Trochus helicites occurs, and 

 its state of preservation. The entire bed is in places quite 

 made up of this form, and always in the state of decorticated 

 casts. To render their appearance more characteristic, the 

 substance of all the casts is riddled with thousands of ex- 

 amples of a little parasitic worm (Spirorhis). 



Although the " Trochus " is found in some of the lower 

 beds of the Lesmahagow section accompanied by Lingida 

 minima, and a bivalve, the so-called Mocliolo])sis JVilssoni, it 

 is in strata approaching nearer to the top of the series that 

 the abundance of molluscan life is met with. 



Combining the results of private workers' researches with 



* Trans. Geol. Soc, Glasgow, 1868, iii., pt. 1, ]). 95. 



t Sil, Brach. of tlie Pentlaiul Hills, p. 22. 



t Salter, Mem. Geol. Survey, Scotl., No. 32, p. 132. 



