70 CRETACEOUS PELECYPODA 



35. Chcenomya, Meek., 1865, (Pal. of Upp. Mo., p. 42, and Proc. Phil. Acad., 

 1865, p. 250). Shell much like Anthracomya, of tliiii structure, more or less com- 

 pressed, posteriorly gaping; hinge with an internal ligament. 



36. Pleuromyo, Agassiz, 181-2, (Etud. crit., p. 231). Shell oblong, equivalve, 

 inequilatei-al, thin, concentrically striated or rilibed, with an oblique process below the 

 beaks of each valve, probably supporting an internal cartilage ; nymphse prominent 

 lor the attachment of an external ligament ; sinus very deep and usually angular {vide 

 Terquem, Observat., &c., p. 54). Ter quern has sufficiently proved that Agassiz' 

 Pleuroniij a is a good genus, and must be kept separate from Homomya, Myacites, and 

 Fanopcea, which it externally greatly resembles. The species are both cretaceous 

 and Jurassic, but only in a few instances has the internal structure of the shell been 

 as yet examined. Anoplomya, Krauss, 1813, (Nov. Act. Acad. Cses. Leop-Carol, 1855. 

 vol. xiv, pt. II, p. 445,) evidently is a synonym of Fleuromya ; it is based upon 

 specimens in excellent preservation, and Krauss gave a very good description of 

 the genus. At the time of the publication of his name the last author could 

 scarcely have suspected the identity of his shell with Pleuromya, so insufficient 

 were the generic characters given by Agassiz; but since Terquem publislied his ob- 

 servations on the " Etudes critiques," the matter has become quite clear. Agassiz 

 also described a number of species under the name Myo^ms, but Pictet and Cam- 

 piche state that all those occurring in the cretaceous beds at St. Croix belong to 

 FanopcBa and not to Fleuromya or Myopsis. Terquem has shown that in some 

 instances (referring to the Jurassic species) both genera are identical. 



37. Gonyomya, Agassiz, 1842, (Etud. crit., p. 1). Shell oblong, rather com- 

 pressed, thin, surface marked about the middle of the flanks with angularly bent 

 striae or ribs ; hinge without teeth ; a slight incision below the beaks and slightly 

 thickened nymphae beyond the same for the attachment of a ligament are present. 

 The genus is only known from Jurassic and cretaceous deposits. 



38. Fholadohiya, Sow., 1823. Shell oblong, thin, pearly inside, in front occa- 

 sionally very much inflated and truncated, posteriorly always gaping, surface 

 marked with ribs radiating from the beaks ; hinge with a small transversally 

 elongated tooth in each valve, adjoining to which there is a small pit ; ligament 

 short, external. 



This genus includes a large number of species, variable in form, but always 

 possessing the characteristic radiating ribs. It is very largely represented in meso- 

 zoic rocks (most numerous in the jurassics), but decreases considerably in the 

 tertiary strata, and only one species was found living in the West Indies. A group- 

 ing of the fossil Fholadomyce., such as that proposed by Agassiz, is of great con- 

 venience in the determination of the species, and ought to be carried out thoroughly, 

 but very good materials are required for such work, because the form of many 

 species is not the natural one, but is often produced by accidental pressure of the 

 rocks in which they are imbedded. When quoting the cretaceous species of 

 Fholadomya, I shall point out those species which belong to other related genera, 

 such as Goniomya, Homomya, and others. 



