Trematode Parasites of British Birds. 265 



and slightly overlapping the ventral sucker. It is on the 

 same level as the ovary. The pars prostatica is also of con- 

 siderable size ; it lies directly in front of the ventral sucker, 

 obscured to some extent by the yolk-glands. 



From the foregoing it is apparent that this species, while 

 bearing a close resemblance to G. bursicola, Odliner, has at 

 least half a dozen constant features of difference. This 

 species has some claim to be considered as the adult of the 

 pearl-forming Trematode of Mytilus edulis. Odhner* has a 

 note on this subject in which he criticizes Jameson's f 

 observations. He shows that the specimens which the latter 

 obtained from Oidemia nigra and identified with Distomum 

 somaterice, Levins., belong, at least in part, more probably to 

 Gymn. bursicola. He also corrects Jameson's obvious error 

 that such small specimens as he found could be the adults of 

 the large cercariae in the mussel. He considers Gymn. bursi- 

 cola as the most probable adult of these eercariie, and there 

 is no reason why this should not be the case. The claims, 

 however, of this new s])ecies appear equally strong. Its 

 dimensions allow for a fair increase in size in attaining 

 maturity, while the immature specimens obtained in Oidemia 

 fusca are only a little larger than the cercariae in Mytilus. 

 The difficulty % of proving the identity of the sporocyst stage 

 in Cardium edule and Tapes pidlastra with the cercariie in 

 Mytilus still remains, and it is quite possible that they may 

 be distinct. There must be at least sis or seven distinct 

 cercarifc of this type to correspond with the number of species 

 of the genus Gymnophallus. 



Genus Maritrema, gen. nov. 



Body flattened, leaf-like, more or less elongated oval, 

 sometimes tongue- shaped. Anterior end usually somewhat 

 more pointed than posterior end. Cuticle provided with 

 minute scales over a considerable extent. Suckers small 

 and nearly equal ; the ventral sucker is situated about the 

 middle of the body. The intestinal diverticula may be short 

 and sac-like or more elongated and narrow, but in no case 

 do they extend beyond the testes ; the bifurcation takes 

 place nearer the ventral than the oral sucker. The excretory 

 system (in Maritrema yratiosum) consists of a small vesicle 

 from which two narrow tubules run forward ; in front of 

 the intestinal bifurcation they begin to widen and bend in 



* Fauna Arctica, iv. (2) p. .312. 

 t Proc. Zool. Soc. l;>02, i. m^. 1.51-lGO. 

 X Vide Ann. & Mag. Nat. llist. (7) xvii. p. 151. 

 Ann. t£- Maq. N. Hist. Ser. 7. I'ol. xx. IS 



