ART. 9. A. NEW TREMATODE FROM WHITE BASS VAN CLEAVE. 3 



traction. As a result, not only body length and diameter show ex- 

 treme variability but the relative position of internal organs and 

 superficial structures display just as surprising instability. The 

 series of drawings in figure 1, A to G, well illustrate the characteristic 

 differences in body size and shape and also show individual peculiari- 

 ties in the relative position of the acetabulum and in the arrangement 

 of the testes. Diagnosis of genus and of species are rendered ex- 

 tremely difficult because of the instability of so many characters in 

 these worms. In the following description of the genus it is entirely 

 possible that some of the characters cited as of generic significance 

 may prove to be of only specific value. The description of any 

 monotypic genus is usually subject to emendation upon the dis- 

 covery of additional species. 



Fkj. 1. — Series of outline drawings, all to same scale, showing relative size and 



BODY FORM OF INDIVIDUALS OF ALLACANTHOCHASMUS VAEIUS. RELATIVE POSITION OF 

 ACETABULUM AND TWO TESTES SHOWN IN EACH DRAWING. 



ALLACANTHOCHASMUS, new genus. 



Diagnosis. — Small distomid trematodes living in the digestive 

 tract of fresh-water fishes. Body highly variable in form. Oral 

 sucker prominent, surrounded by a single complete circle of spines; 

 the entire oral region strongly resembling that of the genus Acantho- 

 chasmus Looss. Entire body surface covered with minute, closely 

 applied cuticular spines. Acetabulum in fully extended or com- 

 pletely contracted individuals almost in center of ventral surface. 

 Genital orifice anterior to acetabulum (fig. 2.) Prepharynx and 

 esophagus lacking. Vitellaria along lateral margins of body an- 

 terior to testes and grouped around intestinal crura, variable in ex- 

 tent of distribution. Testes almost lateral in some contracted speci- 

 mens, more frequently obliquely situated in the body and in some 

 instances almost one directly behind the other. Ovary lobed, occu- 

 pying practically the entire width of body anterior to the testes and 

 posterior to the acetabulum. 



20107— 22— Proc. N. M. vol. 61 10 



