BELLER'OPHON (probably from its re semblance to the helmet of Bellerophon. See BELLEROPHON). A genus of extinct gastropods of the order Aspidobranchiata. This genus is the type of the family Bellerophontid, which is closely related on the one hand to the Plenroto mariidce and on the other to the Fissurelliche. The genus contains about 60 species, of which number one-half are American, the remainder European and Asiatic, and which range through rocks of Ordovician to Permian Age. The shells are globose, symmetrieally coiled in a single plane, with the whorls rapidly enlarging, and the aperture slightly expanded with a thickened margin. The outer margin of the aperture is somewhat sinuate, and is provided with a small notch or slit, which is homologous with the slit of Pleurotomaria, and which served for the passage of the siphons of the living animal. Ex tending backward from the slit along the me dian line is the 'slit-band,' which in some species is replaced by a keel. Although the genus ap
peared at an early period, it did not attain any considerable prominence until Carboniferous time, when it became quite abundant, and in the rocks of which age it is now a characteristic fossil. The best-known American examples are Bellerophon crassus and Bellerophon precari vat us, both common in the eoa l-measures of the Mississippi Valley and the Southwest. A great number of species were formerly included under the name Bellerophon, but more careful study has resulted in their distribution under several new generic names, and even under other family names. Thus the well-known American species, Bellerophon bilobatus, so common in and char acteristic of the rocks of the Trenton epoch, has been proved to be a member of another genus, Protowarthia. See C4ASTROPODA.