1825.] Philosophical Transactions for 1824, Part IL 225 



compression ■^, The results of the chronometrical observa- 

 tions are therefore as much as could be expected in accordance 

 with the correct determinations of the figure of the earth." 



XX. Of the Effects of the Density of Air on the Rates of 

 Chronometers. By George Harvey, FRSE. &c. : communicated 

 by Davies Gilbert, Esq. VPRS. 



See above ; and Annals for May, 1824, p. 392. 



XXI. A Letter from L. W. Dillwyn, Esq. FRS. addressed tcr 

 Sir Humphry Davy, Bart. PRS. 



We gave in our seventh volume, p. 177, Mr. Dillwyn's former 

 letter on the interesting subject of the geological distribution of 

 fossil shells, and the facts in the history of the creation which it 

 indicates : we now extract his present communication. 



" I beg leave to offer to the Royal Society some further obser- 

 vations on the relative periods at which different families of 

 testaceous animals appear to have been created, and on the 

 gradual approximation which may be observed in our British 

 strata, from the fossil remains of the oldest formations to the 

 living inhabitants of our land and waters. 



" The series of strata beginning with transition lime and 

 ending with has, contains shells belonging to various genera of 

 conchifera, cephalopoda, annehdes, and herbivorous tracheli- 

 poda; and also some other shells, as for instance, the multilocu- 

 lar and spiriferous bivalves, which cannot be referred to either 

 of those natural orders, or groups of genera, into which all the 

 other testacea, both recent and fossil, have been divided. In 

 the simple bivalves belonging to these strata, the marks which 

 best serve to distinguish their families are generally obliterated, 

 and but little more can with any certainty be observed, than that 

 the two orders into which Lamark has divided them, have 

 existed together throughout every formation from transition 

 rocks to the present day. An examination of the few perfect 

 specimens which I have met with, however, leads me to suspect 

 that all the dimyaria of these strata have the ligament external, 

 and consequently, that internal ligaments were confined to the 

 monomyairia, till after the lias had been deposited. 



" In the secondary beds above the lias, all the shells may be 

 referred to some of our now existing orders of animals, and the 

 extinction of the unknown orders is immediately followed by the 

 first appearance of another order of mollusca, to which Lamarck 

 has hmited the name of gasteropoda, and, as was first suggested 

 to me by Mr. Miller, all those fossils of the older strata, which 

 have been supposed to be inside and outside casts of patellso, 

 were obviously formed in the concave sides of the vertebra, or 

 by the intervertebral cartilages of a fish. As a fe>v of the carni- 

 vorous trachehpoda are said to have been found in the oolites, 

 their first appearance may probably be referred to the same 

 epoch ; but 1 have not myself been able to detect either of the 



2^ew Series y \i.n.. yi, q 



