. Chordate morphology. Morphology (Animals); Chordata. been separate. These ossifications were joined by a ventral band of perichondral bone. In some acanthodians the mandible ossified as a single unit and bore teeth. The palatoquadrate and mandible had rays extending into the opercular fold. The hyoid arch was modified. The epihyal was quite broad (laterally compressed) at its dorsal end, which may have articulated with the posterior end of the cranium, and tapered to its ventral end. The ossification of hyoid, and branchial arches, was subperichondral. The ventral end of the epihyal cartilag
. Chordate morphology. Morphology (Animals); Chordata. been separate. These ossifications were joined by a ventral band of perichondral bone. In some acanthodians the mandible ossified as a single unit and bore teeth. The palatoquadrate and mandible had rays extending into the opercular fold. The hyoid arch was modified. The epihyal was quite broad (laterally compressed) at its dorsal end, which may have articulated with the posterior end of the cranium, and tapered to its ventral end. The ossification of hyoid, and branchial arches, was subperichondral. The ventral end of the epihyal cartilage articulated with the cartilaginous end of the ceratohyal. The latter was long and slim and abutted anteriorly against a long hypohyal, which in turn articulated with a small median basihyal. There is no evidence of a pharyngohyal. There were three to five branchial arches. The more an- terior arches with pharyngo, epi, cerato, and hypo segments; the more posterior arches lacked the pharyngo and perhaps the hypo segments. The pharyngobranchials were long and with distinct suprabranchial processes. Genera) observations The association of the acanthodians with the sharks is based upon such features as their denti- tion and the separate outer gill openings. The teeth are not unlike those of other fishes, while the gill openings are not truly comparable to those of the shark. The group is usually described as having a complete mandibular gill slit—a con- dition described by the term aphetohyoidea. In the shark this gill slit is reduced to the spiracle. On the basis of the course of the preopercular canal in Acanthodes, Stensio (1947) assumed that these fishes had a spiracle and were not marginal (supratemporotabular) central (postparietal) postorbitol ^ (intertemporol) pineal (parietal preorbital ^ (nasal), rostral (internasal)^ (medial rostral (lateral rostral)' (vomer inferognothal. (dentary) aphetohyoideans. Hydrolagus has a scarcely altered hyoid arch, yet lacks even a spira
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