ACCELERATION AND RETARDATION 965 



from rounded and smooth in youth ; through ribbed, and tubercled, 

 with angular and, later, keeled whorls ; to old age, which was 

 marked by a complete loss of all ornamentation. The late Pro- 

 fessor Alpheus Hyatt was, however, the iirst to recognize the sig- 

 nificance of these changes and to point out that they recapitulated 

 the adult characteristics of successive ancestors. The number of 

 recognizable characters of which the development may be studied 

 is exceptionally large in the Ammonite shell. Thus there is the 

 degree of coiling or involution, which varies from the condition in 

 which the whorls do not even touch each other through whorls in 

 contact and whorls impressed on each other to complete involution, 

 in which the last whorl covers all the preceding ones. Then there 

 is the form of the cross-section of the shell and the character of 

 the outer or ventral surface of the shell, which varies from rounded 

 through angulated to various degrees of channeled and keeled. 

 Again the surface ornamentation varies from smooth to ribbed, 

 noded, or even spinous, and, finally, and in many respects most 

 significantly, there is the progressive change in the complexity of 

 the septal sutures, from simple in the young to often highly complex 

 in the adult. In addition to these, the form and position of the 

 siphuncle often show a definite variation, which may be of consider- 

 able importance. To give a concrete example of the changes in 

 the individual development of the shell and the correlation of the 

 various stages with the adult stages of ancestral forms, we may 

 select a closely related series of ammonites of the family Placenti- 

 ceratidcc, all of which are characteristic of the Cretacic formations 

 of North America. The changes here are chiefly in the form of the 

 cross-sections and the characters of the surface ornamentations. 

 The most advanced form of the series is Stantonoceras pscudo- 

 costatum Johnson, a large, robust ammonite with a broad, rounded 

 venter and rather ill-defined, coarse, rib-like elevations on the lat- 

 eral surfaces of the whorls. When this form is broken down it is 

 found that the next inner whorl has a flattened ventral band bor- 

 dered by a row of faint elongated nodes on either side and a large 

 row of tubercles on the ventrc-lateral angles. At this stage the 

 species has the characters of the adult Stantonoceras guadalnpcr 

 (Roemer), which may be regarded as an immediate ancestor. A 

 still earlier whorl shows a very narrow flattened venter, with a 

 strong row of elongated nodes on either side of the flattening, the 

 surface being otherwise smooth. This corresponds to more primi- 

 tive species, Placcnticcras intcnncdium Johnson, and P. planum 

 Hyatt, one or the other of which was probably in the direct line 

 of ancestry. At a still earlier stage in these shells the venter is 



