AMMONOIDEA. 



861 



turreted spiral, being composed of volutions which pass obliquely 

 round a central axis, an umbilicus being present or absent. The 

 shell is usually sinistral, but is sometimes dextral. The surface 

 is adorned with ribs or nodosities ; and the lateral lobes of the 

 suture-line are symmetrically divided. In Turrilites proper (fig. 



Fig. 780. — Hamites rotundiis, restored. Cretaceous. (Gault.) 



782) the volutions of the shell are in contact, whereas in Helico- 

 ceras the shell is coiled into an open spiral, the coils of which do 

 not touch. Certain of the forms which have been placed in the 

 genus Heteroceras (such as H. polyplocuni) agree in the form of 

 their sutures with Turrilites, from which they 



only differ in the fact that the terminal portion 

 of the shell is coiled into an open spiral, the 

 last whorl sometimes being prolonged in a 

 straight line. All the types in question are 

 Cretaceous, and range from the Neocomian to 

 the Chalk. 



Lastly, the genus Baculites is included in the 

 family Lytoceratidce from the form of its suture- 

 lines, the superior-lateral lobes being symmetri- 

 cally divided. The shell in this genus (fig. 



783) is in the form of a straight elongated 

 cone, with a long body-chamber, the aperture 

 of which is simple and is prolonged ventrally. 

 In some forms a divided " Aptychus " has been 

 shown to exist. The species of this genus 



range from the Neocomian to the highest beds 



Fig. 781. — Ptychoceras 

 Ewericianam, reduced 

 in size. Neocomian. 



of the Chalk. 



Family 10. PtychitiD/E. — This family in- 

 cludes forms of " Ammonites," of variable shape, the spirally 

 coiled shell being flat or ventricose, with a wide or narrow um- 

 bilicus. The body -chamber occupies from two -thirds to three- 

 fourths of a volution. The sutures are sometimes simply angu- 

 lated and resemble those of the Goniatites ; or the saddles are 



