424 PROCEEDINGS OP THE ACADEMY OF [1881. 



Hippagus, with the original specific name modified into Verti- 

 cordius. Both tlie English and the Italian species have very 

 little in common with Lea's Hippagus, which is edentulous, and 

 belong properly to the genus Verticordia (Trigomilina of 

 D'Orbigny), as reconstituted by conchologists. 



Another singular fossil, long known to pahieontologists as the 

 Chama ? arietina of Brocchi (Conchiologie Fossile Subapennina, 

 ii,p. 668), and which systematists generally referred to Isocardia, 

 was thought by Sismonda (S3aiopsis Method. Anim. Invert. Fed. 

 Foss., p. 18; fide Homes, Die fossilen Mollusken des Tertiar- 

 Beckens von Wien, ii, p. 169) to be referable to the genus 

 Hippagus of Lea, but the dcntiferous conformation of the hinge 

 did not escape the attention of Meneghini, who, in 1851 (Con- 

 siderazioni sulla Geolog. Stratigr. della Toscana, p. 180), con- 

 stituted it into the genus Pecchiolia, restoring to it the original 

 specific name of argentea, proposed, in 1*797, by Mariti. Deshayes 

 in (about ?) 1860 (Animaux sans Vertebres, Bassin de Paris, i, 

 p. 809), described a minute fossil from the Paris basin under the 

 name of Hippagus Z^eanMS, which, in the prominence and recurved 

 nature of its beaks, to some extent recalls the Hippagua isocardi- 

 oides of Lea, but which differs in the presence, in each valve, of a 

 cardinal tooth. 



Deshaj'es was apparently doubtful as to the true generic 

 position assigned to his species, inasmuch as he states that a 

 more complete study of the American shell may lead to the 

 separation of the two species into distinct genera. Having 

 shown the correctness of Lea's figure and description, by the 

 discovery of the allied Indian form, Stoliczka proposes (Palseon- 

 tologia Indica, Cretaceous Fauna, iii, p. 225) the generic name 

 of AUojjagus, for the species from the Paris basin, which name it 

 ought to retain. It will tlius be seen that fossil shells belonging 

 to no less than four distinct genera have been alternately referred 

 to the American genus Hippagus. 



All these agree, more or less, with each other in the closed and 

 nacreous or semi-nacreous shell, recurved umbones, simple pallial 

 impression, and the internal or subinternal arrangements of the 

 ligaments. The^^ differ in the dentiferous character of the hinge. 

 The opinions of naturalists have been greatl}' at variance as to 

 the position to be assigned to these genera in a natural classifica- 

 tion, and, indeed, there appears to be no small difficuly in 



