398 A. IT. Foord — Ammotiites in the British 3Tuseum. 



(Upper Cretaceous) may be cited as examples of this small group. 

 (Table-cases 63, 64.) 



The Haplooeratid^, ranging from tbe Inferior Oolite to the 

 Middle Chalk, differ in many respects from the HARPOCEUATiDiE, 

 from which they branch off. Tlie shell is generally thick, some- 

 times remarkably so, as in Pachydiscus peramplus of the Middle 

 Chalk; there are also periodic constrictions or grooves upon the 



Fig. 16. 



Siephanoceras Blagde^ii, Sow. (Inferior Oolite.) 



surface, which gave rise to the old group-name Ligati. Finally, the 

 contour of the edge or periphery of the shell is uninterrupted, there 

 being no keel such as is met with in the Harpoceratid^. There is 

 an aptychus in a few forms. Of the flatter kinds of shells of the 

 present family, Desmoceras ligatum (Fig. 15) of the Neocomian and 

 Desmoceras planulatum of the Gault may be referred to as examples. 

 (Table-case 64.) 



Fig. 17. 



Siephanoceras coronatiim, d'Orb. (Callovian.) 



The last section to be described is that of the Stephanoceratid^, 

 an extensive and varied group of shells which are characterized in 

 many cases by the symmetrical and beautiful ribbing with which 

 they are ornamented. A few examples will serve as illustrations, 

 viz. Siephanoceras Blagdeni (Fig. 16), Inferior Oolite, Siephanoceras 

 coronaliim (Fig. 17) and Cosmoceras Jason (Fig. 18), of the Callovian 



