Dr Johnston on the genus Caligus of Leach. 
since the angles approach in every instance within one degree tq 
the limits of 120° and 90°, and the regular composition very ofv 
ten hinders the crystals from being observed on all sides, while 
the inclination of the optical axes of no polarisation upon each 
other is very considerable, and easily ascertained. 
The inferences drawn from Dr Brewster’s general law, re^ 
specting the existence of two polarising axes in crystallised sub- 
stances, had excluded the forms of axotomous lead-baryte from 
the rhombohedral system, previous to their correct determina- 
tion, and even in contradiction to the opinions entertained by 
crystallographers. The preceding demonstration, that they are 
hemiprismatic, reconciles the results of both sciences. 
Art. XVI. — Notice respecting the genus Caligus of Leach. 
By George Johnston, M. D., Extraordinary Member of 
the Boyal Medical, and Corresponding Member of the Me- 
dico-Chirurgical Societies of Edinburgh. 
SIR, 
r AM not aware that any naturalist has heretofore suspected, 
that there exists a considerable difference in the external structure 
of the sexes, of at least one species of the curious genus Caligus. 
My observations have led me, however, to believe, that this is 
the case ; and should they appear to you as new as they are to 
me, this notice may, perhaps, be deemed not unworthy a place 
in the Edinhitrgh Philosophical Journal. 
In the autumn of 1822 I found, on the common cpd-fish, what 
I considered at the time a non-descript species of Caligus. A few 
weeks since, I met with the same animal, but intermixed on the 
body of the fish, with the Caligus Mulleri of Leach ; and on a 
comparative examination, their resemblance in general form, and 
more particularly in the form of the clypeus, antennas, and legs, 
was so close as to suggest, that they might be but the male and 
female of the same species. I felt more confirmed in the accu- 
racy of this conjecture, on finding in the only three perfect spe- 
cimens of the C. Mulleri I had procured, two little packets of 
eggs, attached beneath to the lower end of the large segment of 
