Br. J. Allan Thomson — Brachiopod Morphology. 71 



6. Conclusions. 



1. From the distribution of the radio-active elements among igneous 

 rock-types and the variation of igneous rock-types in depth, it is 

 deduced that the radio-active elements are specially concentrated in 

 the earth's crust, the density of distribution being greatest near the 

 surface, and falling off in depth. 



2. Direct experiments show that radio-active disintegration, and 

 therefore the consequent heat generation, are unaffected by tem- 

 peratures of 2500° C. and by pressures of 160 tons to the square inch, 

 conditions which correspond with those of a depth of 50 miles (80 km.) 

 within the earth. 



3. Thermal considerations demand, as an alternative to inhibition 

 (which as shown in 2 cannot be assumed), that the radio-active 

 elements should be almost altogether confined to an outer shell of 

 the lithosphere. 



4. Volcanic temperatures indicate that the density of distribution 

 of the radio-active elements falls off rapidly with depth, the rate of 

 decrease being perhaps greater under the continents than under the 

 oceans. 



5. For mathematical purposes radio-activity and heat generation are 

 assumed to decrease exponentially in depth, this being a law of 

 decrease which can be adapted to agree with the limiting conditions 

 of rock distribution (superficially and in depth) and volcanic 

 temperatures. The case chosen for further mathematical treatment is 

 discussed in the preceding section, case 4 (b). 



(Part II will appear in the March Number.) 



III. — Brachiopod Morphology : Types of Folding in the 

 Terebratulacea. 



By J. Allan Thomson, M.A.- D.Sc, F.G.S. 



IN descriptions of Recent and Tertiary Terebratulids at least, the 

 type of folding is often hardly mentioned, and is left to be 

 inferred from the figure, in spite of the fact that Douville 1 in 1879 

 showed that it might be even of generic importance. Buckman 2 has 

 more recently drawn attention once more to the importance of this 

 character and has shown that its due consideration may lead to 

 fruitful results in classification. The object of this paper is to point 

 out further cases in which it has been neglected. 



Shells showing no folding, with a rounded anterior margin and 

 a plane commissure, are said by Buckman to be in the 'lenticular 

 stage'. Three primary modes of development of such a form are 

 possible ; it may become — 



1. Dorsally uniplicate, i.e. with a single fold on the dorsal valve 

 opposing a sinus in the ventral valve, or 



1 H. Douville, "Note sur quelques genres de Brachiopodes (Terebratulidre 

 et Waldhemiidae) " : Bull. Soc. geol. Fr., ser. in, torn, vii, pp. 251-77, 1879. 



2 S. S. Buckman, "Brachiopod Morphology: Cincta, Eudesia, and the 

 Development of Bibs " : Quart. Journ. Geol. Soc, vol. lxiii, pp. 338-43, 1907. 



