On the basis of the morphology of specimens identified by B. H. Ransom, it is confirmed that Ostertagia trifurcata Ransom, 1907, has a typical accessory bursal membrane with two slender supporting accessory bursal rays and that Ostertagia pinnata Daubney, 1933, is a synonym of the former species. These conclusions are contrary to those of some recent authors. A historical review of the pertinent literature is given along with the nomenclature used by different authors. Spicule lengths, determined by measuring specimens recovered from domestic and wild ruminants, are given for Ostertagia ostertagi, 0. circumcincta, 0. occidentalis, 0. trifurcata, and Teladorsagia davtiani. The range and average length of the spicules for each species deviates considerably from those previously reported, which greatly reduces the importance of this character in diagnostic keys. Following other workers who reported Teladorsagia davtiani Andreeva and Satubaldin in Skrjabin, Shikhobalova, and Shul'ts, 1954, in ruminants in their countries, including Jansen (1958) in the Netherlands and Rose (1960) in England, Becklund (1962) reported the hosts and geographic distribution of this parasite in the United States. His information was obtained by examining specimens deposited in the National Parasite Collection as Ostertagia trifurcata Ransom, 1907, for the presence or absence of a typical accessory bursal membrane, a character used by Ransom (1907) in describing the genus Ostertagia. Specimens not having this character were redetermined as Teladorsagia davtiani. More recent reports by other workers propose changes in the nomenclature of the aforementioned two species. Drozdz (1965) proposed a new combination Teladorsagia trifurcata (Ransom, 1907) and synonymized Ostertagia trifurcata and Teladorsagia davtiani with it. More recently, on the basis of drawings by various authors published over the last 60 years, Lancaster (1969) reached the same conclusion. Because Ostertagia trifurcata is a parasite of domestic sheep and is therefore frequently mentioned in the literature, the present study of Ransom's specimens and illustrations was undertaken in an attempt to alleviate the confusion surrounding this species. Received for publication 7 July 1970. * Deceased. In connection with this study, it was noted that the spicule measurements of 0. trifurcata often fall considerably outside the range quoted in the original description. A similar discrepancy was observed in other species of Ostertagia. Most veterinary reference texts and compilations on the trichostrongylids indicate measurements identical to, or differing only slightly from, those in Ransom (1911); very little additional original work has been published. Since spicule size is used by some workers as a key diagnostic character, the spicules of four common species of Ostertagia plus Teladorsagia davtiani from various host species were measured to determine more accurately the range of intraspecific variation. MATERIALS AND METHODS All specimens were obtained from the National Parasite Collection where they had been preserved in a solution of 92 parts 70% ethyl alcohol, 3 parts formalin, and 5 parts glycerine. A few specimens, apparently cork-stained, were so dark they were bleached by the Mayer's chlorine method involving KC103 and HC1. All were cleared for study in pheno-alcohol or lactophenol and placed in 95% ethyl alcohol before being returned to the preserving fluid for storage. Cover glasses were supported by human hairs or cover glass fragments to protect critical specimens. All specimens were examined that were identified as Ostertagia trifurcata by B. H. Ransom, or those that might possibly be this species which were collected by E. C. Stevenson (collector of several type-series specimens of Ransom). In addition, the spicules of five species (Table II) of medium stomach worms were measured, and the range and average length determined for each species. A total of 451 specimens was