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THE AMERICAN NATURALIST [Vol. XL 



flat, cartilaginous copula (C.) of the first and second visceral 

 arches. It is flat, broadly heart-shaped in outline, and is com- 

 posed of soft, fibro-cartilage. It is united around its periphery 

 with the inner border of the hyoid arch by a thin but tough con- 

 nective-tissue membrane. On the midventral line of this copula, 

 at its posterior edge, in a slight cartilaginous elevation to which 

 are attached the anterior ends of the first and second visceral 

 arches, the former to its lateral, the latter to its posterior border. 



The first visceral arch (F^) is composed of two slender, some- 

 what flattened, S-shaped bars united in the median line by the 

 copula that has already been described. Each bar of this arch 

 is composed of a single rod of firm, translucent cartilage. Along 

 its entire median border this arch is united, by a tough, fibrous 

 membrane, to the outer border of the second visceral arch (V^.) 

 which, in distinction to the preceding arches, is composed almost 

 entirely of bone. Its constituent bars, instead of being S-shaped, 

 are practically straight laterally though somewhat arched in a 

 dorso- ventral direction. Viewed from the ventral aspect, the two 

 bars of this arch form an almost perfect letter V. Each bar is 

 formed of two elongated bones, of which the anterior or dorsal 

 one is the longer. Both bones are nearly circular in cross section, 

 and are enlarged at each end, the two adjacent ends being the 

 larger, and somewhat flattened. The free end of the posterior 

 or ventral bone is tipped by a small piece of cartilage. 



Between the two bones of the second visceral arch, on each side, 

 is a small ])a(l of c artilage which extends medially and becomes 

 spread out for the attachment of the anterior ends of the third (F^) 

 and fourth visceral arches, of which the former is composed 



almost entirely of bone, and the latter entirely of cartilage. The 

 third and fourth arches are formed of distinct bars; that is, they 

 are not united in the midventral line by a copula as are the pre- 

 ctMling arches. The bars of the third and fourth arches on each 

 aiv united wirh each other at both ends, and form a loop 

 which ^fivc- to >tiHVii the border of the permanent gill slit. The 

 third bai- i-^ of bone, and i> tipped at the posterior end with a 

 small head of cartilau'e for attachment to the corresponding end 



is slightly curved, so that the loop is kept permanently open, while 



