CEPHALOPODA 89 



These constrictions, often accompanied by a prominent 

 rib, undoubtedly indicate periods of rest, when the Ammo- 

 nite ceased for a while to grow. They may be traced in species 

 belonging to other groups, as well, e.g., in bi]jlex and triplicatus, 

 as in the Ligati; but most frequently all indications are obli- 

 terated by subsequent growth. It has been a question whether 

 the lateral processes of Ammonites Duncani (fig. 21, 2), are 

 formed and removed periodically, or whether they are pecu- 

 liar to the adults, and mark the close of their outward 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 Argo- 

 naut) in those of adult size and condition. 



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-whirl of 

 the full-grown shell. 



Varieties of form, such as marked the palaeozoic NautiliclcB, 

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

 their reign. The Baculite (fig. 21, 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 Hamitcs the shell is straight, 

 returning upon itself after a certain space, and forming a simple 

 or complex hook. In Ptyclwceras 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 Scaphites 

 (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. Heli- 

 coceras, again, connects the last with the Turrilite by its 

 elevated spire terminating in a prolonged crozier. 



