1874.] 237 [Hyatt. 



structure of the parts. These are apt to occur with a certain fixity 

 of succession which enables the observer to predict with considerable 

 certainty the general characteristics of the succeeding forms of any 

 given group after he has thoroughly studied the development and 

 succession of a few of the lowest. They correspond to what natura- 

 lists are in the habit of calling parallel forms, often also representa- 

 tive forms. These forms begin in every group with which I am ac- 

 quainted with a certain low or open-whorled form and evolve, in course 

 of time and by inheritance, more and more involved whorls, or else the 

 whorls are modified in the characteristics which usually accompany 

 the normal increase of the involution, namely, by the increasing thin- 

 ness of the shell laterally, flattening of the sides which become more 

 and more convergent outwardly, and the tendency of the abdomen to 

 become narrower. This and the origin of most of the groups from 

 certain single ancestral species of the discoidal or open-whorled forms 

 show conclusively that these forms arise independently in each group. 



But it must be noticed that they can be only thus limited in each 

 group or series of groups which are genetically connected. The 

 range of forms comprehend every imaginable modification of the 

 original inherited or stock form of the "third stage among Ammonites. 

 This is tubular or coniform and has an inherited tendency to grow 

 by increasing the abdominal more than the dorsal side, thus revolv- 

 ing upon itself. Therefore while the choice or selection of the origi- 

 nal forms by which a series starts into being is practically unlimited 

 except by the possibilities of the typical discoidal form of the em- 

 bryo and young, the subsequent development in each series becomes 

 more and more limited according to the size of the group. The same 

 law of inheritance which renders the embryonic form of the third 

 stage fixed or invariable in each individual of the true Ammonites 

 of the Jura, subsequently accomplishes the same purpose, to a less 

 degree and with greater fluctuation, for the later developed forms and 

 characteristics of each separate series or group, obliging them to 

 evolve, if they progress at all, a certain succession of forms which 

 have been described above. 



It will be noticed that I use the word progress in a special 

 sense as applicable to a certain class of parallel forms .and not to 

 those with which we shall presently deal, the old-age forms, which 

 though equally perfect in the phenomena of parallelism, cannot be at- 

 tributable to growth. The former are the mechanical results of the 

 growth or increase in size of the sliell of the common embryonic form 



