968 



'PRINCIPLES OF STRATIGRAPHY 



modern species is succeeded by one in which strong spines occur, 

 caused by periodic notchings or emarginations of the shell margin 

 along the line of the angulation. The tubercles and spines pass the 

 one into the other by what appears to be a process of enlargement 

 of the tubercles. 



When we come to consider the series of forms which lead up 

 from the tubercled (Tertiary) species (F. fusifonnis, F. tubercu- 

 latum) to the modern form, we find that certain intermediate 

 characteristics have been omitted. As shown by the specimen of 



Fig. 256. Development of the gastropod shell (Fulgur and S3'cotypus). a. 

 Protoconch of Sycotypus caiialicitlattis. b, c. The same before 

 hatching, showing smooth shell; animal with velum. (The early 

 stages of Fulgur caricum are identical with these. ) d. Fulgur 

 fusiformis. e. F. rapum (representing F. maximmn). f. F. 

 tr it 0)1 is. 



F. fusiformis figured (Fig. 256, d), the last part of the last whorl 

 has already lost the tubercles and has become smooth and rounded 

 in outline. This is prophetic of the form next to be noted, F. maxi- 

 mmn (Fig. 256, e). In this shell the tubercled stage is passed through 

 quickly — a case of acceleration in development — and the smooth, 

 rounded whorl stage makes up the greater part of the shell. Thus 

 the normal characters of F. fusifonnis have become condensed to 

 a few whorls, in this manner making room for the smooth whorl 

 which characterizes the shell. It is in certain advanced accelerated 

 individuals of this type that the emarginate spines so characteristic 



