THEORY OF RADICALS AND MORPHOLOGICAL EQUIVALENCE. 29 



numbers whenever conditions became unfavorable to the evolution of normal 

 progressive forms. The degenerative nature of the uncoiled Ammonitinaa and 

 Lytoceratinse of the Cretaceous has been very generally recognized. They were 

 regarded as diseased forms by Von Buch and Quenstedt, and Neumayr's dis- 

 covery of the prevalence of simpler sutures even in the normal forms of the Cre- 

 taceous has completed this wonderful picture of wholesale degradation. It can 

 be confidently stated, that the well known cretaceous genera of uncoiled shells, 

 Crioceras, Ancyloceras, Ptycboceras, Hamites, and Baculites, are the morpho- 

 logical equivalents of similar forms occurring earlier in the Jura, but that they 

 are not their lineal descendants. The series of Gosmoceras (Amm.) bifurcatum 

 worked out by Quenstedt, 1 and studied also in detail by the author, had shells 

 which became gradually uncoiled. Quenstedt named the uncoiled forms Ham- 

 ites, but has correctly traced them to the coarsely tuberculated species Cos. 

 bifurcatum. There is also a finely tuberculated specimen, baculatus, with a 

 broader abdomen, which does not otherwise differ from bifurcatum. To this 

 last he is disposed with good reason to refer an arcuate and a straight baculites- 

 like shell. This same tendency is observable among the shells of the Planorbidas 

 at Steinheim. 2 Among living shells of a closely allied, if not identical, species of 

 Planorbis at Magnon, 3 similar but exaggerated and evidently diseased forms 

 occur, and the physical conditions are such that we can attribute the tendency to 

 the unfavorable and abnormal nature of the surroundings. 



We have previously pointed out, that such uncoiled shells could not have 

 had the same habits as closely coiled ones. The appearance of a rostrum 

 in the Ammonitinse indicates that they had become exclusively crawling ani- 

 mals, in consequence of the disappearance of the ambulatory pipe or hyponome. 

 In the shells of uncoiled Ammonitinae the rostrum though smaller is still present. 

 Scaphitoid, ancyloceratoid, hamitoid, and ptychoceratoid shells, to whatever gen- 

 era they may be eventually referred , have one peculiarity in common, the liv- 

 ing chamber is bent backwards, forming a shepherd's crook. The absence of the 

 hyponome and the presence of the rostrum in these forms show that they could 

 not have been swimmers, like the modern Nautilus with its large hyponome and 

 corresponding sinus in the aperture and in the striae of growth along the outer 

 (ventral) side of the whorls. The shepherd's crook added to the rostrum in the 

 living chambers of the shells mentioned above indicates not only a wide departure 

 in habits from the close-coiled Nautiloids, but also from the close-coiled Ammo- 

 nitinae, since such creatures could not have crawled with facility. They must have 

 been stationary, either hanging among the branches of plants and feeding upon 

 them, or living with their lower portions buried in the ground and cleaning the 

 surrounding surfaces for their food. Other suppositions might be made, but all 

 hypotheses would involve a wide departure from the habits of their immediate 

 ancestors, and from those of their morphological equivalents, Lituites, Gyroce- 

 ratites, or other uncoiled Nautiloids, none of which have the reversed shepherd's 



1 Der Jura, p. 400, plates lv., Ixxii.; Amm. Schwab, Jura, p. 576, plate Ixx. 



2 Gen. of Plan, at Steinheim, Summ. PI. ix. 



3 Ann. Soc. Malacol, Brussels, VI., 1871, Planorbis complanatus (forme scalaire), by M. Lois Pire. 



