THEORY OF RADICALS AND MORPHOLOGICAL EQUIVALENCE. 31 



differ but little at any age. The four or six lobes of the young are retained 

 throughout life, and have comparatively simple margins. The adult, however, 

 is not similar to a true larval form except in the same sense that an old whorl is 

 similar to its own young, or the toothless gums of an old man are similar to the 

 same parts before the teeth appear. The Baculites are not as a rule strictly 

 tubular whorls, as in the nsepionic stages of other Ammonitinte, but are gener- 

 ally more or less compressed in the adults, like the aged whorls of close-coiled 

 shells. The development skipped the normal progressive stages of the proximate 

 close-coiled ancestors, and, like syphilitic children, these shells had no proper 

 adult stages, but assumed senile, degradational characteristics while still young, 

 and are, as we have said above, purely nostologic forms. 



The law of evolution for geratologous forms seems therefore to be as follows: 



The response or reaction of the forms of different series to the action of the ordinary 

 surroundings in the same habited produced retrogressive morphological equivalence, ivhen 

 the external influences ivere unfavorable to growth. 



We cannot account for the number of uncoiled Ammonoids in the Upper Cre- 

 taceous, their wide distribution, and the undeniable fact, that they were the 

 members of an order then rapidly nearing extinction, unless we imagine the gen- 

 eral conditions of life during this period to have become unfavorable. The 

 unfivorable causes produced in the forms of the groups as a whole similar modifi- 

 cations to those caused by the unfivorable effects of the local surroundings in Cos. 

 bifurcaium, and other shells in more limited localities during the Jurassic period. 



The bifurcatus shells and the uncoiled cretaceous Ammonoids are not isolated 

 individuals, — like the turrillitic deformities of Ammonitinse figured by D'Orbigny 

 under the name of Turr. Boblagi, Valdani, and Coynarii, or the planicostan 

 deformities figured in Cor. rotiforme, Plate III. Fig. 7-13, and the scalariform 

 Planorbidaa of Magnon, or multitudes of similar instances known to every student 

 of these fossils, — but series of varieties, species, and genera. These can only be 

 accounted for as the result of hereditary tendencies acting upon races and species, 

 through successive generations, for periods of time more or less prolonged. 



The evidence is very strong, that Baculites, Scaphites, etc. of the Cretaceous 

 are not necessarily species of the same genus, but probably always polyphylettic 

 in origin. The Baculites of North America have so close resemblance to those 

 of Europe, that they are usually considered as allied species ; but there are 

 indications in the peculiar nodular markings and great size of many species, 

 which lead us to think that they originated from American stocks. Several 

 species of American Scaphites have common characteristics in the sutures, and 

 in the aspect of the ribs and tubercles, and the abdomen, which suggest affinity 

 with Placenticeras. Meek's plates of Scaphites, published in his Invertebrate 

 Paleontology, 1 exhibit common characteristics so far as the sutures are concerned, 

 especially the large size and length of the first lateral lobes, but he gives no 

 figures of the tuberculated young of Placenticeras placenta, which make this com- 

 parison closer. The Amm. Mullanus on Plate LVIII. Fig. 1, 1 a, from Upper Cre- 

 taceous, Chippeway Point, near Fort Benton, has exactly the form in some 



1 U. S. Geol. Survey of Territ., Hayden, IX., plate xxxiv., and Placenticeras on plate xxiii. 



