34 



Guide to the Invertehrata. 



6ALLEBT The extreme delicacy of the ramifications of the sutures in this 

 species excels that of any other Ammonite known. The leaf-like 

 terminations of the sutures in the Phylloceeatid^ are the dis- 



11 



tinguishing feature in this group ; they are well seen in the 

 typical species, Phylloceras heterophyllum, Fig. 59. This family 

 hegan in the Trias, but it extended through the Jurassic into the 

 Cretaceous. A large example from the Chalk near Brigliton may be 

 seen between Wall-cases 3 and 4, measuring 44 inches in diameter. 

 The suture-line is again the most important feature in the 

 family next in order, viz. the Lytoceratid^, for it supplies 

 the justification for connecting together an assemblage of genera 



Fig. 59. — Phylloceras heterophi/llum, Shy. 

 Upper Lias : Whitby. 



Fig. 60. — Lytoceras Jimhriatum, Sby. 

 Middle Lias : Charmouth. 



Wall- 

 case 11. 

 Wall-case 

 9, Table- 

 case 61. 



differing widely in external shape. The family begins in the 

 Trias, and is largely represented in the Jurassic and Cretaceous 

 rocks. One of the most beautiful species of this group is Lytoceras 

 fimhriatum (Fig. 60), whose regular, wavy lines running across 

 the shell make it very attractive to the eye ; the effect being 

 heightened by the bold, sharp, transverse ridges encircling the shell 

 at frequent intervals, representing the former "lips" of the shell. 

 In Macroscaphites the shell is for about four convolutions or whorls 

 exactly similar in shape to a Lytoceras, when it suddenly takes 

 a direction tangential to the coiled part, and after pursuing a nearly 

 straight course for a short distance it bends back in a hook-like 

 termination (Fig. 61). In Hamites the shell is bent at both ends, 

 the apical or smaller (initial) end being again bent : thus the shell 

 has three curvatures. Owing to its slenderness the apical part 



