A new aspidogastrid trematode species, Cotylogasteroides barrowi, is described from the intestinal tract of the freshwater mussels Lampsilis siliquoidea and L. ventricosa. This species is clearly distinguished from C. occidentalis (Nickerson, 1902), the only other member of this genus, by a smaller number of alveoli on the ventral holdfast organ, smaller body size, posterior position of the reproductive organs, lack of a seminal receptacle, and lack of a papilla on the oral disc. Diagnostic characteristics of the genera Cotylogaster Monticelli, 1892, and Cotylogasteroides Yamaguti, 1963, are reviewed, with modifications to the latter. Mussels were collected from the Cuyahoga River in April 1971, at Hiram Rapids, Ohio, and maintained in chilled (15 C), aerated stream water. All soft parts of the mussels were dissected microscopically, and all worms found were kept in a medium of half Ringer's solution and half mussel blood for further study. After observations and measurements of the worms were complete, they were fixed in AFA with slight pressure, stained with Delafield's hematoxylin, Semichon's acetocarmine, or Semichon's acetocarmine with iron, and permanently mounted by standard methods. Three specimens were fixed in cold Lillie's neutral buffered formalin and subjected to the Koelle and Friedenwald (1949) indoxyl acetate method to demonstrate nonspecific esterase activity. Measurements are in microns (micrometers) unless otherwise indicated. Thanks are expressed to the Hiram College Department of Biology for furnishing space and equipment required for collecting in that area. Appreciation is also extended to R. J. Etges for assistance in the preparation of Figure 4. Cotylogasteroides barrowi sp. n. (Figs. 1-4) Description (based on 10 mature specimens): Body 2.35 to 4.30 mm long by 0.96 to 1.35 mm wide at midbody with conical dorsal protuberance posteriorly; narrow anterior portion of body completely retractable into the larger posterior portion, tegument aspinose; ventral holdfast organ about % as long as body, wider anteriorly, with 14 to 17 transverse and 50 to 52 marginal alveoli, between which prominent marginal organs are situated; mouth surrounded by muscular, heart-shaped Received for publication 5 October 1971. oral disc; prepharynx 250 to 400 long; pharynx subspherical, 250 to 360 by 200 to 250; intestine saccate, simple, reaching posteriorly to level of testes; excretory system symmetrical, the 2 primary ducts arising at dorsal lobe of oral disc, terminating posteriorly at excretory bladders; 2 excretory pores open subterminally at base of dorsal body protuberance; testes paired, posterior, subspherical, 120 to 190 in diameter, obliquely tandem with dextral testis anteriormost; vasa efferentia 250 to 600 long; vas deferens 1.20 to 1.80 mm long, with glandular cells around distal end near genital atrium; genital pore ventromedial, between pharynx and anterior margin of ventral holdfast; cirrus and cirrus pouch absent; ovary 90 to 140 wide by 160 to 250 long, situated immediately in front of anterior testis; Laurer's canal passes from oviduct to posterior opening between 2 excretory pores; uterus slightly undulating, passing posteriorly from ootype to rear of worm, ascending to midbody, looping to rear of worm, and finally ascending to genital pore; Mehlis' gland present; seminal receptacle absent; eggs unembryonated, 50 to 100 by 20 to 50; vitellaria paired, tubular, asymmetrical, forming horseshoe-shaped arch extending 1/3 of body anteriorly; nervous system with massive ganglia at base of oral disc in which extensively branching fibers are seen; dorsal and ventral pairs of nerve trunks with numerous transverse connecting fibers from oral disc to posterior end of body; paired nerve fibers serving each marginal organ. Named in honor of Dr. James H. Barrow of Hiram College. Type specimens: USNM Helm. Coll. holotype No. 72127; paratype No. 72128.