8 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 81 



present in the uterus of one specimen. These small specimens, how- 

 ever proved to be Megalodiscus teiwperatus^ by comparison with 

 young individuals from other hosts. 



Family DICROCOELIIDAE Loess, 1907 



Genus MESOCOELIUM Odhner, 1911 



This genus is known by many species from Asia, Africa, and 

 Australia, but so far as I am aware there is no previous record from 

 America. 



MESOCOELIUM AMERICANUM, new species 



Plate 1, Figure 2 



Specific diagnosis. — Mesocoelimn: Body length 1.2 to 2 mm, maxi- 

 mum width 0.5 to 0.7 mm. When properly relaxed before fixing and 

 not flattened, the body is widest in the region of the intestinal fork, 

 rounds anteriorly, and tapers gradually posteriorly. The cuticula is 

 thin, and in the cephalic region it contains numerous short spines. 

 The oral sucker is subterminal and nearly circular in outline. The 

 diameter varies from 0.21 to 0.27 mm. The acetabulum in young 

 specimens lies at the end of the first third of the body, but because 

 of the distention of the posterior region with eggs it is relatively 

 more cephalad in the older specimens. It is 0.13 to 0.2 mm in diam- 

 eter. The ratio between the acetabulum and the oral sucker varies 

 somewhat, but usually falls between 3 : 5 and 3 : 4. The prepharynx 

 is very short and in whole mounts is often obscured. The pharynx 

 is nearly globular and measures 63/x to 105;u, in diameter. It is very 

 close to one-half the diameter of the acetabulum. The esophagus 

 is short, seldom equaling the diameter of the pharynx. The ceca 

 curve sharply laterad, then turn caudal and run parallel to the lateral 

 margins. In young specimens they nearly reach the middle of the 

 body, but in fully matured specimens they do not extend far beyond 

 the end of the first third of the body. The genital organs lie close 

 in the fork of the intestine. The ovary is posterior to the testes on 

 the left side, and its cephalic margin nearly always lies anterior to 

 the posterior margin of the acetabulum. It is somewhat irregular 

 in shape but is usually more or less ovoid, with the tip directed 

 medio-caudad. It varies from 0.084 to 0.092 mm to 0.14 by 0.18 mm. 

 The ootype and Mehlis's gland lie medio-caudad of the ovary. On 

 the caudal margin of these structures there is a small yolk reservoir ; 

 at this point a seminal receptacle emj^ties and Laurer's canal leaves. 

 The seminal receptacle is a simple sac lying posterior to the yolk 

 reservoir. Laurer's canal runs medio-caudad, loops back on itself, 



