257 



NOTES ON TWO LARGE AMMONITES FROM THE 

 HOLDERNESS DRIFT. 



C. THOMPSON, B.Sc. (Lond.)- 



(plate XII.). 



In following the life history of an individual ammonite, as 

 ■exemplified in the shell, from the earliest stage to the last, it 

 will be seen that as a rule, in infancy, the whorls are quite 

 smooth. This stage continues through a variable number of 

 whorls, which number seems to be a characteristic of the genus 

 to which the specimen belongs. Then in the youthful stages 

 it acquires its characteristic ornaments, gradually advancing 

 from obscure folds, or swellings, to the more ornate costae, 

 spines, etc. The acme of ornamentation may be said to be 

 reached in, and to persist through the adult stage. In old age, 

 however, senile decay is manifested by a gradual loss of orna- 

 ments, and the whorl becomes either smooth, or traversed by 

 slight ridges, mere indications of the former ornate folds. Again 

 the shape and amount of involution of the whorl changes, 

 ■especially in old age. 



If we now examine a series of species, the successive repre- 

 sentatives of the genus, or race ; we find that the changes men- 

 tioned begin at earlier and still earlier periods in the lives of 

 successive species descended from some given progenitor. 

 Thus in a series A, B, C, D, we may find in the adolescent stage 

 of B the characters of the adult of A, its predecessor ; that in 

 ■C and D, one or more stages may be crowded out altogether 

 from their individual development, so that even in youth they 

 may resemble the aged representatives of A, or, have acquired 

 entirely new characteristics. 



This mutation of form and ornamentation is expressed by 

 Hyatt's Law of Acceleration, {vide his ' Genesis of the Arie- 

 tid«'). 



Perhaps one of the most striking, or most manifest examples 

 of the law, is the species acceleratum of the genus Aster ocer as. 

 This sjiecies has been shewn by Hyatt to have descended from 

 ohtusum, through stellare, and to display in its adult stage the 

 characters of the last stage of the other two. It is a rare 

 ammonite, but a few specimens have been found in the Cote 

 <i'Or and also in southern Germany. These are preserved in 

 the museums at Semur and at Stuttgardt respectively. 



agio July i. 



