A. H. Foord — Ammonitefi in the British Museum. 397 



wliicli it loses in the adult, while the umbilicus, at first narrow, 

 becomes ultimately wide. (Wall-case 10.) 



The Harpooeratid^. are derived from the Arietid^. The oldest 

 forms begin in the Middle Lias, and they extend into the Inferior 

 Oolite. The shells are flattened and keeled, and have falciform or 

 sickle-shaped ribs, or strise. Eildoceras hifrons, from the Upper 

 Lias of Whitby, shows this kind of ornamentation very distinctly. 



Fig. 14. 



Grammoceras serpentinus, Schloth. (Upper Lias.) 



Leioceras opalinum (Lower Lias) is another characteristic species, 

 the aperture with ear-shaped lateral processes. Grammoceras serpen- 

 tinus (Fig, 14) characterizes the " Serpentinus zone," or Jet Kock, 

 which has yielded some of the finest and best preserved Ammonites 

 of the Yorkshire Lias. Of Inferior Oolite species Ludicigia MurcMsoncB, 

 Sow., and Oppelia suhradiata, Sow., may be mentioned. (Wall- 

 case 10.) 



Fig. 15. 



Desmoceras ligatum, d'Orb. (Neocomian.) 



The PuLCHELLiD^ (Cretaceous) are remarkable for the simplicity 

 of their suture-line, some resembling Goniatites, others Ceratites, in 

 this respect. They are considered to be true Ammonites, descendants 

 of the Jurassic Harpoceratid^, but in a state of degeneracy. Pul- 

 cliellia compressissima (Lower Cretaceous) and Neolobites Vibrayeanus 



