Charles Davison — Earthquake Phenomena. 157 



comparison of the size of the teeth, about half the size of the Pleurodns 

 affinis, Agass., also from the Coal-measures. The teeth may be 

 distinguished by the deep transverse ridges and the absence of 

 •widely expanded lateral margins, as well as their elongate form. The 

 specimens described in the Transactions of the Eoyal Dublin Society 

 from the collection of the late Mr. Wood, of Richmond, were tabulated 

 as having been found in the Mountain Limestone of Richmond, in 

 Yorkshire. I have since found that they were from the Upper Yoredale 

 beds near Leyburn, the same locality from whence the collection of 

 Mi*. Home was obtained ; consequently Mr. Parker's specimens from 

 Derbyshire are the only ones which have been found in the Lower 

 Limestone. An example of the spine of the fish has not been found ; 

 the teeth do not differ in any appreciable degree from the types, and 

 they are probably the same species. 



Inserts sedis. 



A large number of small palatal teeth have been found associated 

 with those already described. They are of varied forms, usually 

 more or less elongated, sometimes of uniform width and even and 

 smooth on the surface ; in others, the median portion is raised and 

 gibbous, the lateral extremities depressed and attenuated in width. 

 The under-surface is devoid of basal or root-like extension, and is 

 frequently more or less concave, thus conforming to the convexity of 

 the crown. They are enamelled and covered with pustulations or 

 punctations where the surface has been worn. 



Many of these small teeth are probably immature examples of 

 species already described ; others may have been associated with 

 them in different portions of the jaws, whilst it is possible that some 

 may be distinct and independent species. It is considered advisable 

 for the present to await the advent of other examples which shall 

 render the determination of their affinities and relationships some- 

 what less problematical. 



III. — On the Occurrence of Undisturbed Spots in Earthquake- 

 shaken Areas. 1 



By Charles Davison, M.A., 

 Mathematical Master at King Edward's High School, Birmingham. 



ONE 0^ the most important discoveries in Seismology is that 

 which proved the close connexion in position between earth- 

 quake-centres and lines of fault, leading up to the view that the 

 earthquake is an effect and not a cause of fault-formation. It may be 

 the momentary result either (1) of the sudden fracturing of the 

 earth's crust, or (2) of the impulsive friction that must accompany 

 the relative displacement of the rock-masses adjoining the fissure. 

 Now, earthquakes are frequent at places where faults are already 

 advanced in the process of formation. They also recur continually 

 at one and the same place along a line of fault, the number felt in 

 1 A note read before the Birmingham Philosophical Society on March 11, 18S6. 



