PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 267 
BRANCHIAL SYSTEM. 
The branchial apparatus has five very short arches and six clefts, the 
arches being fringed with a double row of lamelle, with cartilaginous 
axial supporting filaments. The very short branchial arches seem to 
have been backwardly displaced, an the clefts open downward instead 
of laterally. The clefts lie in a pair of anteriorly divergent furrows in 
the floor of the pharynx. The skeletal elements of the branchial ap- 
paratus are probably in large part cartilaginous and imperfectly devel- 
oped. The branchize are covered by a soft integumentary fold, in 
which there are no apparent traces of branchiosteges. 
MYOLOGY. 
The muscular apparatus which actuates the jaws and hyomandibular 
suspensor in Gastrostomus presents one very remarkable feature in that 
its cross-section apparently exceeds that of the dorso-ventral lateral 
muscular masses of the nape. These muscles operate (1) the mandible 
and (2) the hyomandibular and quadrate. | 
The mandible is extended by a powerful extensor muscle, lying be- 
hind and external to the hyomandibular. (Its belly, in our largest 
specimen, is about as long as that of the hyomandibular—about .035™, 
and its filiform tendon is partially ossified, and measures .065™ in 
length, thus together constituting a total length of .10™.) Its origin 
is in the lateral cranial fosse in the pterotic, and its insertion into the 
angular portion of the mandible, which is turned upward and backward 
over the distal end of the quadrate, like the olecranon of man. This 
muscle may be called the extensor of the mandible, and is practically 
placed posteriorly to the suspensor. 
The other principal muscle originates anteriorly, externally and in- 
ternally to the articulation of the mandibular suspensor, which depends 
obliquely backwards from the skull, and its belly is about .05™ long 
and eight times the bulk of the mandibular extensor. Part of it 
passes down between the palatine and the suspensor internally, and its 
internal belly is prominent within the mouth, while its external belly 
is strongly marked from without, behind and below the eyes, imme- 
diately behind which its anterior origin from the skull begins. The 
dentigerous palatine is really anterior to it, but closely apposed, while 
a furrow in the posterior side of its belly receives the upper half of the 
suspensorium. Its origin largely covers the postorbital portion of the 
ventral face of the cranium nearly as far back as the basisphenoid and 
from the external ventral margin of the skull behind the eye to near the 
middle line. Its insertion seems to be partly into the angular portion 
of the mandible in front of its articulation with the quadrate and partly 
into the quadrate itself. Its function is to close the mandible and to 
divaricate and approximate the greatly elongated suspensor, which 
itself has a cartilaginous joint near its upper third, and enjoys a special 
mobility in virtue of its peculiar articulation with the cranium. 
