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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