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 Ammonitine and 
Lytoceratinee of the Cretaceous has been ver 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, Ptychoceras, 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 Cosmoceras (Anum.) bifurcatum 
worked out by Quenstedt,’ 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. 
bfurcatum. 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 Planorbidx 
at Steinheim.? Among living shells of a closely allied, if not identical, species of 
Planorbis at Magnon,® 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 Ammoniting 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 Ammonitine 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 striw 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- 
nitine, 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. Pl. ix. 
8 Ann. Soc, Malacol, Brussels, VI., 1871, Planorbis complanatus (forme scalaire), by M. Lois Piré. 
