40 



Guide to the Invertehrata. 



GALLERY sometimes remarkably so, as in Pachy discus peramplus of the Middle 

 "^^' Chalk ; there are also periodic constrictions or grooves upon ihe 

 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 Haepocekatid^. 

 'J'here is an aptychus in a few forms. Of the flatter kinds of 

 shells of the present family, Desmoceras ligatum (Fig. 71) of the 



Fig. 73. — Stcphanoceras coronatum, D'Orb. Callovian. «, side-view 

 b, keel view of shell. 



Table- l^eocomian and Desmoceras planulatum of the Gault, may be 

 case 64. referred to as examples. 



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



Fig. 74. — Cosmoccras Jason, Rein. 

 Callovian, or Oxford Clay. 



Fig. 75. — HopUtes radlatus, Brug'. 

 Neocomian. 



an extensive and varied group of shells which are characterized 

 in many cases by the symmetrical and beautiful ribbing with 



