. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. 180 Btdletin Museum of Comparative Zoolof^y, Vol. 135, No. 3. Fig. 9. Chimaera monsfroso. Muscles of the head, lateral view. , Maxillary cartilage; , prelabial car- tilage; , premandibular cartilage; , levator anguli oris anterior; , levator anguli oris posterior; , levator of prelabial cartilage; , adductor mandibulae; , labialis anterior muscle; , lobiaiis inferior muscle; , labialis posterior muscle; , preorbitalis muscle; , nasal capsule. (
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. 180 Btdletin Museum of Comparative Zoolof^y, Vol. 135, No. 3. Fig. 9. Chimaera monsfroso. Muscles of the head, lateral view. , Maxillary cartilage; , prelabial car- tilage; , premandibular cartilage; , levator anguli oris anterior; , levator anguli oris posterior; , levator of prelabial cartilage; , adductor mandibulae; , labialis anterior muscle; , lobiaiis inferior muscle; , labialis posterior muscle; , preorbitalis muscle; , nasal capsule. (Adapted from Luther.) fusions which took place within the visceral and cranial skeleton during the independent evolution of the Holocephali. As the gill arches became compressed under the oc- cipital region and the extrabranchial carti- lages spread to form an opercular cover, the branchial constrictor muscles gave way in favor of an expanded hyoid constrictor sheet. The branchial levators, adductors, and interbranchials all became reduced in accordance \\'ith the reduction and com- pression of the cartilages of the arches. Since the mandible is short in holocepha- lians and forms only a shallow curve, the ventral portion of the hyoid constrictor (which reaches the midline in sharks as the interhyoideus) apparently shifted the origin of its most anterior fibers forward to the connective tissue on the posterior ventral edge of the mandible. There being no division between the palatoquadrate and the ethmoid region of the cranium, the muscles innervated by the trigeminal nerve spread over the entire anterior region of the head. The divisions of this muscle which insert upon the labial cartilages would seem to be late developments. If the branchial muscles of the Holocephali evolved as suggested here, it would be logical to seek an ancestral stock in which the shortening of the head region had already begun. The ptyctodonts show such a condition and may thus be a better choice as a
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