410 WILLIAM NICOLL. 
Olsson mistook the point beyond which the ova could not be 
seen for the termination of the vagina—a not at all unlikely 
mistake. On such a supposition my specimens agree very 
well with those of Olsson, otherwise the difference is of more 
than specific value.! On this matter Linton says nothing 
definite, but, his figures show the genital aperture well 
forward in the neck, always single and sometimes with 
exserted cirrus. Careful measurements of his drawings, 
which we may assume to be drawn in proportion, show that 
the genital aperture is nearer to the posterior end of the 
pharynx than to the anterior border of the ventral sucker. 
In fig. 30, where the pharynx is not represented, the genital 
aperture is about ‘19 mm. behind the posterior border of the 
oral sucker. 'Theaverage length of the pharynx being *16 mm. 
it is evident that the genital aperture is not far behind its 
posterior border, 
The fact that Linton represents the genital aperture 
single tends to cast some doubt on my observation of the 
absence of a sinus genitalis, The exserted cirrus in Linton’s 
specimens might have obscured the double aperture, but it 
is possible that the condition as I observed it was purely 
accidental. Against thisis the fact that it occurred in several 
specimens. 
The cirrus-pouch is, as already mentioned, of the charac- 
teristic Distomid shape, with a wide posterior part containing 
the vesicula seminalis, gradually narrowing into a slender 
anterior part containing the terminal part of the ductus 
ejaculatorius. The latter usually passes straight back from 
the genital aperture, and the thicker part is bent on it either 
to the left or to the right. It extends back to the level of 
the centre of the ventral sucker. It occasionally does not 
| By the kindness of Dr. Jiigerskidld I have been enabled to examine 
two of Olsson’s original specimens from Thymallus vulgaris and 
to confirm the correctness of the above suppositions. The genital 
aperture (or closely apposed apertures) is situated midway between the 
two suckers and therefore not far behind the end of the pharynx. Its 
position is probably most correctly described as midway between the 
suckers. 
