640 DAVID HILT TENNENT. 
and G. vipers, as well as giving new figures of G. 
gracilescens and G. crucibulum. 
Olsson (14, 1875) redescribes : 
1. G. gracilescens, Rud.; embryonic forms from brain of 
Gadus minutus, adult from Lophius piscatorius. 
2. G. armatum, Molin, correcting Molin’s error with 
regard to the genital aperture. 
3. G. crucibulum, Rud. 
= Monostomum crucibulum, Rud. 
= Distomacrucibulum, Dujardin. 
= Gasterostoma crucibulum, Diesing. 
and gives a figure of the last-named form. 
Rud. v. Willemoes Suhm (15, 1873) expresses the opinion 
that G. crucibulum=G. armatum. He gives figures of 
the embryos of these forms, and thinks that Bucephalus 
haimeanus is a stage in the life history. 
Levinsen (16, 1881) redescribes G. armatum, confirming 
Olsson (13) in his observation as to the single genital 
aperture. 
Linton (17, 1899) describes and figures : 
1. G. ovatum, Lt. (“ Monostomum orbiculare,” Rud. Lt. in 
‘Proc. U.S. Nat. Mus.,’? Vol. xx, p. 541) from Lobotes 
surinamensis. 
2. G. arcuatum, Lt., from Sarda sarda and Carcha- 
rinus obscurus. 
3. Gasterostomum sp. from Scomberomorus maculatus. 
4, Gasterostomum sp. from T'ylosurus marinus. 
The form which is to be considered in the present paper is 
that designated by Linton (17) as Gasterostomum sp. from 
Tylosurus marinus. 
For reasons which can best be discussed in a later section 
of this paper, I shall tentatively consider the form as 
G. gracilescens, Wagener. 
