106 PALEONTOLOGY. 



obliterated by subsequent growth. It has been a question 

 whether the lateral processes of Ammonites Duncani (fig. 31, *), 

 are formed and removed periodically, or whether they are 

 peculiar to the adults, and mark the close of their out- 

 ward growth. The first conclusion is more probable from 

 analogy; and they are commonly found with small and 

 apparently young shells, but not (any more than the lateral 

 spines of the living Argonaut) in those of adult size and con- 

 dition. 



It was remarked by the elder Sowerby that Ammonites 

 were most beautiful when of middle growth, the ornamental 

 characters being less developed in the young, and lost in the 

 adult. The ribs and spines, and even the keel or furrow of 

 the back disappear, in many instances, from the body- whorl of 

 the full-grown shell. 



Varieties of form, such as marked the palaeozoic Nautilidm, 

 are met with in the Ammonitidw, chiefly towards the close of 

 their reign. The Baculite (fig. 31, 4), with its straight shell, 

 is characteristic of the upper chalk ; and the Turrilite, which 

 is spiral, and usually a left-handed spiral, abounds in the 

 lowest beds of the same formation. In Hamites the shell is 

 straight, returning upon itself after a certain space, and 

 forming a simple or complex hook. In Ptychoceras these 

 limbs of the hook-like shell are in close contact. The 

 Toxoceras is curved like a bow; in Crioceras the discoidal 

 whirls are separate ; and in Scwphites (including Ancyloceras) 

 the shell, at first compact like an Ammonite, or open-whirled 

 like Crioceras, lengthens out finally, and returns upon itself 

 like the crozier of the Hamite. Helicoceras, again, connects 

 the last with the Turrilite by its elevated spire terminating 

 in a prolonged crozier. 



Of these forms, Ancyloceras alone is found in the oolites; 

 all the rest are cretaceous; and most abound in the alpine 

 districts of the south of France. 



