| Summary: | The thesis consists of two main sections and deals with taxonomic studies of the family Paramphistomidae Fischoeder, 1901. The first section reviews the various schemes of classification published for the group. The various characters both morphological and histological, employed in the classification of members of the family are discussed and assessed. The value of tegumental papillae as specific characters is recognised and scanning electron micrographs of the different types observed on the anterior end of some species are provided. The second section is the systematic part and deals with discussion of ten genera occurring mainly in ruminants. The history of each genus is given and the species are described, illustrated and their synonymies discussed. The genus Paramphitomum Fischoeder, 1901 is redefined and only nine species, including one as new are considered valid. The genera Liorchis Velichko, 1966 and Srivastavaia Singh, 1970 are synonymized with Paramphistomum Fischoeder, 1901. The genus Calicophoron Näsmark, 1937 is redefined and revised to contain only 12 species some of which in new combinations. The genus Bothriophoron (Stiles and Goldberger, 1910) Gretillat, 1958 is considered a synonym of Calicophoron Näsmark, 1937. The genus Gigantocotyle Näsmark, 1937 is redefined and contains only four valid species. The subgenus Explanatum Fukui, 1929 is raised to full generic rank, redefined to contain only three valid species. The genus Cotylophoron Stiles and Goldberger, 1910 is revised to contain only seven valid species. The genus Orthocoelium (Stiles and Goldberger, 1910) Price and McIntosh, 1953 is revised, only nine species are considered valid and two new species are added, one of the latter is appended as a publication. The genera Ceylonocotyle Näsmark, 1937 and Cochinocotyle Gupta and Guta, 1970 are considered synonyms of Orthocoelium (Stiles and Goldberger, 1910) Price and McIntosh, 1953. Two new genera are erected and appended as publications. These are Bilatorchis which contains a single species described as new and Leiperocotyle which contains three species, two in new combinations and one as a nomen novum. Of the three species assigned to the genus Stephanopharynx Fischoeder, 1901 only one, S. compactus Fischoeder, 1901 is considered valid. The one and only species of the genus Balanorchis. B. anastrophus Fischoeder, 1901 is redescribed, illustrated and for the first time, its histological features are elucidated. Keys to the species of the above genera are provided and scanning electron micrographs of the tegumental surfaces of some species are shown. The zoogeographical affinities of the paramphistomids of ruminants are discussed and a list of the hosts and localities of the species is given.
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