276 Mr. W. King on the Tetrabranchiate Cephalopods. 



became decidedly more numerous, and even the sides of the pos- 

 terior lobes which verge on the dorsal aspect of the shell assumed 

 this character, as may be observed in the Ceratite* ; and subse- 

 quently, that is, throughout the Jurassic and Cretaceous epochs, 

 the whole contour of the posterior, and also of the anterior lobes, 

 became digitated, which is shown in the beautiful foliations of 

 the Ammonite. 



It has been previously remarked that the plain-edge-plate Ce- 

 phalopods or Nautilidiansf of the primary period ran into a va- 

 riety of shapes, from a close coil to a straight line, — a disposition 

 which we never find displayed in the early AmmonidiansJ, as 

 the Goniatites, nor in their successors, the Ceratites : but no 

 sooner do the Ammonites appear than they imitate the forms of 

 their remote predecessors : thus a certain section of the last 

 disunites its coils and becomes the gyroceroid Criocerus § ; this 

 before long completely unfolds itself, and thus we have the cyr- 

 thoceroid Ammonocerus ||, whose light and graceful arch is after- 

 wards unbent to form the fragile orthoceroid Baculite. But our 

 list of changes still remains unexhausted : another section of the 

 Ammonites retains the normal form for a certain time; then strikes 

 off its coil at a tangent to be afterwards curved back, and hence 

 we have the Scaphite ; and the Criocerus, as if conscious that it 

 could improve this figure, assumes the more pleasing yet singular 

 form of the Ancylocerus. 



But these forms are merely modifications of the shell unfold- 

 ing its coils on one and the same plane. Unlike the primary 

 shells in this respect, certain of the Ammonidians are obliquely 

 coiled or spiral, and the coils strike off from the slight deviation 

 exhibited in the Jurassic Turrilites Boblayei to the extreme which 

 is observable in the Cretaceous T. costatus. 



The Turrilite is essentially an Ammonite having a spiral con- 

 volution, inasmuch as the coils, in both kinds, are in contact ; but 

 the last is not the only form that passes into a spirally coiled 

 Cephalopod, since we find the evolute Criocerus obliquating its 

 coils to become the Heliocerus. 



* In none of the figures that I have seen of the Ceratite is the character 

 particularized in the text represented : it is displayed in a specimen belong- 

 ing to the Newcastle Museum. 



f Family Nautilidce. % Family Ammonidce. 



§ According to Mr. Morris (Catalogue of British Fossils) a species of 

 Criocerus is found in the Kelloways rock, Wiltshire. 



|| Lamarck's genus Ammonocerus is evidently the same as D'Orbigny's 

 Toxocerus, which is of a later date. D'Orbigny states that it is found in the 

 Jurassic system, but does not mention in which division. I found it in the 

 Jurakalk near Streitberg, Franconia, in 1839. Criocerus must be found in 

 an earlier, or at least an equivalent rock, before what is said in the text of 

 Ammonocerus can be received, even as a sound metaphor. 



