( m ) 
XIII. — Descriptions, Characters, and Synonyms 
of the different Species of the Genus Icarus, 
with Critical and Explanatory Jlemarh'S. 
By W. JVTacgillivray, A. M., 
Member of the Medical Society of Aberdeen, and Corresponding Member of 
the Wernerian -Society, 
{Read 10th January 1824.) 
There is scarcely a genus of birds in which more 
confusion has reigned than in that which forms the subject 
of the present memoir. To those versed in the study of 
birds, and especially to those who have laboured to acquire 
a knowledge of the species which are found in our own 
country, no apology is necessary for an attempt to dissipate 
the uncertainty and doubt which every one experiences on 
comparing the specimens which he may have procured with 
the descriptions of authors. To the student alone, it may 
be necessary to observe, that the species of gulls, forming 
a very natural genus, and being, in many cases, most close- 
ly allied in manners, in colour, and in size, — and, moreover, 
presenting much particular diversity, chiefly with respect 
to plumage at different ages, in the same species,— together 
with a strong general resemblance in diffeient species dur- 
ing the first years, — are very difficult to distinguish by a 
