THEORY OF BAD l> MORPHOLCX JUIVALENt 



numbers whenever condil tme unfavorable to the evolution of normal 



nerative nature of the uncoiled Ammoo itinae and 



1 - has been ignized. They were 



I forma by Von Buch and Quenstedt, and Neumayr's dis- 



■ of the prevalence of simpler sutures even in the normal form- of th< 



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



ufidenth Btated, thai tin- well known cit .•■una of uncoiled sheila, 



' \ - Ptycboceras, rlamites, and Baculitea, are the morpho- 



I equivalents of similar forms occurring earlier in the Jura, but that tli<-y 



are not their lineal descendants. The I 



i out l>\ Quenstedt, 1 and studied also in detail by the author, lia<l shells 

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

 tly traced them to the coarsely tuberculat 

 'inn. There is also a finely tuberculated specimen, baculatus, with a 

 r abdomen, which does not otherwise differ from bifurcation. To this 

 ed with good reason n arcuate and a straight baculites- 



i.-ll This same tendency i- observable among the Bhells <>f the Planorbidsa 

 nheim ' Among living Bhells of a closely allied, if not identical, 

 1'laii" ' tnilar but ted and evidently diseased forms 



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

 • and abnormal nature of 1 1 1 « - surroundings, 

 We b r. sly pointed out, that sia-li uncoiled shells could not have 



ime habits as closely coiled ones. The appearance of a rostrum 

 in the Ammonitinaa indicates that they lia<l become exclusively crawling ani- 



ence of the disappenrani fthe ambulatory pipe or byponome. 



In tli incoiled Ammonitinaa the rostmm though smaller is still present 

 ancyloccratoid, hamitoid, and ptychoceratoid Bhells, to whatever gen- 

 entually referred , have one peculiarity in common, the li\- 

 "•nt backwards, forming a shepherd's crook. The absence of tin* 

 unl 1 1 1 « - | ■• : the rostrum in these forms shew that thej could 



like the modern Nautilus with it* large byponome and 

 nus in the aperture and in the Btriaa of growth along the outer 

 the whorls. The shepherd's crook added t<> the rostrum in the 

 of the Bhells mentioned above indicates not onh a wiuV departure 

 'in the cl -coiled Nautiloids, but also from tb< led Ammo- 



Id not have crawled with facility. They must have 

 nary, either hanging among the branches of plant ding upon 



tin-in. <>r living with tlnir lower portions buried in the ground and cleaning the 

 surround inf i Other suppositions might be made, but all 



1 involve a wide departure from the habits of their immediate 



ind from their t "photo 1 G 



other uncoil. <1 Nautiloids, none of which bav< shepherd's 



