6 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 61. 



repon. In figure 5 the anterior extremity is distinctly truncated 

 with the oral spines arranged about the margin of a flattened area 

 which in its center bears the mouth opening. A rather unusual con- 

 dition is shown in figure 4. Naturally the oral sucker in its func- 

 tional operations undergoes conspicuous movements through the op- 

 eration of its powerful muscles. As a result the sucker may be either 

 broad and shallow or narrow and deep with all intermediate stages 

 between the two conditions. Some of these stages are shown in 

 text figure 1. 



Surface structures such as the acetabuhim and the genital orifice 

 are also subject to considerable modifiability in relative positions as 

 well as general form. While the acetabulum is usually circular in 

 outline it is distinctly elongated in the transverse axis in many pre- 

 served specimens. The position of the genital orifice with reference to 

 the acetabulum is subject to some variability which is probably di- 

 rectly correlated with the muscular state of the immediately sur- 

 rounding region of the body wall. In some individuals the crescentic 

 elevation which bears the genital orifice is directly in contact with 

 the anterior margin of the acetabulum, but still others show these 

 two structures separated by a distance as great as the diameter of 

 the prominence which bears the orifice. That these differences are 

 not due to the mobility of the acetabulum alone is evidenced by the 

 fact that separation of the two structures is just as frequently en- 

 countered in specimens which have an acetabulum perfectly circular 

 in outline as in those with an elongated lateral axis. 



Relationships.— Odhnor (1911:522) has maintained tliat certain 

 genera of trematodes which lack oral spines despite that lack still 

 must belong to the same family as Looss's genus Acanthochasmus. 

 Caecincola and Cryptogonimus^ two genera from fresh-water fishes 

 of North America, are cited in his discussion of this interrelationship. 

 The genus Allacanthochasmus in its fundamental organization rather 

 closely approaches the conditions found in Osborn's genus Crypto- 

 gonimus. It seems possible, then, that the genus AUacanthochasmus 

 provides an additional link in the chain of relationships so strongly 

 maintained by Odhner. A few specimens of Caecincola were encoun- 

 tered as a simultaneous infestation with Allacanthochasmus in the 

 white bass. In these few individuals the extreme development of the 

 uterus prevented thorough study of the internal organization, but all 

 available evidence corroborates the identification of the species as 

 Caecincola parvulus INIarshall and Gilbert. 



Linton (1898:535) described Distomum tenue from the rectum of 

 the striped bass {Roccus lineatus) at Woods Hole, Massachusetts. 

 In some respects AUacanthochasmus varius simulates the conditions 

 in D. tenue, but upon close observation it is readily seen that in 



