NOTES AND MEMORANDA. 395 
have ever been described or depicted; they differ curiously 
in the two species. 
The spermatozoa of P. marinus, notwithstanding the great 
size of the species, are much the smallest, and have a distinct 
and rounded head. Their mean length is about ;,'5q inch, 
and their thickness ;;1,5. They were obtained from a fish 
$2 inches in length and three pounds in weight, taken on the 
12th of May, 1874, in the river Stour, near Sturry Mill, 
about two miles below Canterbury. The milt, which dis- 
tended the whole abdomen from the pericardium to the anus, 
was a soft pulpy mass chiefly composed of a creamy semen, 
and so rich in, and crowded with, spermatozoa of such minute- 
ness that they were with difficulty distinguishable; and it 
was not before the semen had been much diluted and placed 
under Powell and Lealand’s ;!; objective that a good view of 
them was obtained. Under a lower power, especially in the 
pure semen, nothing more than congeries of indistinct 
rounded points appeared, like those which I have described 
in the ‘ Proceedings’ of this Society (P. Z. 8. 1842, p. 99), 
as the “molecules of the semen.” In short, unless great 
care be taken, the spermatozoa in the ripe testis are so very 
faint, minute, and abundant, that they are likely to escape 
detection. 
But the spermatozoa of the little Petromyzon planer: are 
much larger and more easily seen. ‘They are club-shaped, 
without a distinct head, and have an average length of .'o> 
inch, and a thickness of ;;1,;. They were obtained in April 
from a fish 6 inches in length and 2 drachms in weight. 
Further details concerning the generative organs of both sexes 
are given in the paper first quoted in the present communi- 
cation. 
