390 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
margin of the squamosal and the lateral margin of the parietal, (pi. 39, 
figs. 1, 2, Sq. and Pa.) This is shown in the figures of numerous 
works, ( e . g., Huxley, ’64, fig. 61 B; Parker, ’85, pi. 2, fig. 3; van 
Bemmelen, : 01, p. 770, pi. 30, fig. 5, pi. 31, fig. 5; Fischer, : 01, pi. 29, 
fig. 3; van Kampen, : 05, fig. 1; Gaupp, : 05, p. 844, fig. 406, etc.). 
On the other hand the close association of parietal and squamosal 
in the adult has led to what I regard as erroneous views of its homol¬ 
ogies in the non-mammalian vertebrates. It is, however, sufficient 
to say at this point, that there is nothing in the early development 
which necessitates the identification of the mammalian squamosal 
with the bone which joins the parietal in the lower tetrapodous Verte- 
brata (Stegocephala and Cotylosauria). 
The relation of the squamosal to the incus is, in my estimation, 
much more important. The conditions described indicate an intimate 
relation between the two elements in question in the ancestors of the 
Mammalia which has been lost in the latter group. This involves a 
discussion of the homologue or homologues of the incus in the non¬ 
mammalian vertebrates. 
Three widely different views have been advanced upon this point, 
besides some minor variations such as that of Driiner (: 04) which 
scarcely demand attention since they do not affect the main questions 
at issue. These views are:— 
1. The incus is the homologue of the hyomandibular. 
2. The incus is a part of the Sauropsidan columella. 
3. The incus is the quadrate, changed in form and function. 
I am well aware that this is a much disputed question and that each 
of the three views has been advocated by able authorities. The whole 
matter has been reviewed recently by Gaupp (’99) and Kingsley (: 00) 
so that a summary of their results and references to more recent liter¬ 
ature are all that is necessary here. It is indeed hardly necessary to 
deal with the arguments, pro and con, regarding the homologies indi¬ 
cated under 1 and 2 for, if it be shown that the third is correct, the 
others certainly cannot be true. The arguments concerning the third 
are as follows:— 
1. The incus arises from the procartilage of the mandibular (ptery- 
goquadrate-Meckelian) arch; so does the quadrate. 
2. It arises in front of the Eustachian (spiracular) cleft; so does 
the quadrate. The columella and the hyomandibula, on the other 
hand, are postspiracular. 
