588 EDWARD PHELPS ALLIS, JR. 
the inferior ligament as described by Gegenbaur, and I also find 
a superior ligament extending from the cranial wall to the dorsal 
edge of the spiracular canal and hence similar to one of the two 
strands found in Chlamydoselachus. In Lamna I find both liga- 
ments well developed, the inferior one being large and compli- 
cated and prolonged around the ventral end of the hyomandibula 
onto the mandibular cartilages. In Cestracion both ligaments 
are also found, and, in addition, the dorsal edge of the spiracular 
canal is connected with the cranial wall by a feeble strand of 
connective tissue. 
These two ligaments, more or less developed, are thus probably 
found in nearly all, if not all, selachians, and they are such im- 
portant structures that it would seem as if they must have their 
serial homologues in the branchial arches. Related to the dor- 
sal ends of the branchial arches are interarcual ligaments and ar- 
cual and interarcual muscles. These arcual and interarcual mus- 
cles are of two kinds, the interarcuales dorsales I of Vetter’s de- 
scriptions, called by M. Firbringer (97) the interbasales and 
said by Edgeworth (711) to be derived from spinal myotomes, 
and the interarcuales dorsales IJ and III of Vetter’s descriptions, 
called by Fiirbringer the arcuales dorsales and said by Edgeworth 
to be derived from the branchial myotomes. ‘The interarcuales 
dorsales II are short muscles which extend across the angle be- 
tween the epibranchial and pharyngobranchial of an arch, and, 
if this muscle primarily existed in the hyal arch, it could readily 
have given origin to the inferior postspiracular ligament, the 
muscle losing its attachment to the aborting pharyngohyal and 
secondarily acquiring attachment to the neurocranium, and then 
becoming ligamentous. The superior ligament might have been 
derived either from the interarcualis dorsalis I which extended from 
the hyal to the mandibular arch; from the fascia of connective 
tissue that is associated with that muscle in Chlamydoselachus 
and has been already described; or from the corresponding dor- 
sal interarcual ligament. In either case, because of the markedly 
dorsal position of the spiracular canal, this interarcual ligament 
would have been pushed upward toward the lateral wall of the 
neurocranium, and, passing lateral to the vena jugularis, could 
