VISCERAL ARCHES OF THE GNATHOSTOME FISHES. 317 
acquired, by inheritance, from an earlier form would probably 
not have been maintained by Yetter. This cicatrice, important 
as it is, does not involve the dermal tissues, nor are those 
tissues even said to be adherent to it, as the extrabranchials 
are said to be. This, in itself, is singular, as is also the fact 
that whereas the distal fibres of each constrictor, in all the 
Selachii, Acanthias included, always have, throughout their 
entire course through the related branchial diaphragms, a 
course parallel to the free edge of that diaphragm, they are 
shown by Yetter running nearly at right angles into a line, 
whether cicatrice or aponeurosis, that is said to represent a 
part of the former free edge of that particular diaphragm. 
Yetter has himself called attention to this, and has attempted 
to explain it, but the explanation is not convincing. Further- 
more, the conditions in specimens of Suyllium and Mustelus 
that I have examined, and that will be later described, are so 
decidedly opposed to this interpretation of the meaning ot" 
the aponeuroses that I consider it wholly impossible that they 
represent lines where adjoining branchial diaphragms have 
fused with each other, and, in my opinion, the small external 
gill openings of Acanthias are due simply to the retarded 
development of the outer edge of a gill cleft as compared 
with that of the inner edge of the same cleft. 
From the several surfaces of origin above described by 
Vetter, the fibres of each constrictor superficialis of Acanthias 
run at first antero-ventrally, and the proximal (anterior) and 
larger part of them are said to be inserted either entirely 
(Yetter) on the next anterior aponeurosis, or partly also 
(Marion, 1905) on the extrabranchial that underlies that 
aponeurosis. This latter extrabranchial is, according to 
Vetter’s descriptions (loc. c 1 1 . , p. 430), the one related to 
the arch to which the muscle belongs. Hut there is then 
confusion in the descriptions, for as Yetter has previously 
said, as just above stated, that certain of the proximal fibres 
of each constrictor have their origins on the extrabranchial 
of the arch to which the muscle belongs, these fibres could 
not have their insertions on that same extrabranchial. The 
VOL. 62, PART 3. NEW SERIES. 23 
