54 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



and Radiolites, — which appear to constitute together with Hippu- 

 rites a natural and well-defined group, possessing the rank of a 

 Family, in the sense in which that term is employed by the most 



Figs. 30 & 31. — External views o/Diceras and Requienia. 

 Fig. 30. Fig. 31. 



Fig. 30. Diceras arietinum, Lamarck. ^. Fig. 31. Requienia ammonia, Goldfuss. J. 

 a, a, point of attachment ; I, I, ligamental grooves ; ^', posterior-adductor inflection. 



orthodox conchologists. It includes above 80 species, which are 

 found in all four quarters of the world, and never beyond the limits 

 of the Cretaceous strata. 



Like many other groups of animals, it gradually attained a maxi- 

 mum development and then declined. Thus, only 3 species are 

 found in the Neocomian, 13 in the Upper Greensand, 50 in the 

 Hippurite-limestone, and 15 in the Chalk. 



Several families of Lamellibranchiate Bivalves were more abundant 

 in the ancient seas than at the present day. The species of fossil 

 AnatinidcB are four times as numerous as the recent ; and more than 

 half the genera of CyprinidcB and Trigoniadce are lost. But the 

 Hippuritidce form the only instance in which a whole family of bi- 

 valve shells has become extinct. 



Affinities of the HippuritidtE. 



I. In the work of M. Picot de Lapeirouse (1781), the Hippurites 

 are described as Orthoceratites ; the Radiolites as Ostracites. This 

 view was adopted by Mr. Parkinson. 



II. Von Buch, so late as 1840 {Leonh. u. Bronn's N. Jahrh. 1840, 

 p. 573), regarded the Hippurites as corals. 



III. Prof. Steenstrup, of Copenliagen, announced to the British 

 Association (through Prof. Forbes) in 1850 that they were Anellides, 

 allied to the Serpulce cymospirce of Savigny. 



These, however, differ — 1st, in being symmetrical and bi-lateral ; 

 2ndly, they have no muscular attachment to their shell ; 3rdly, the 

 operculum is not articulated ; it is one of two organs attached to the 

 head, of which sometimes one, sometimes the other is developed 



