The elasmobranch fishes elasmobranchfish03dani Year: 1934 THE ELASMOBRANCIl FISHES 45 the walls of the pharynx. In plan the branchial arches are essentially similar to the first two, but in the branchial arches there are typically four segments to every arch; these segments differ among themselves in minor details. The mandibular or first arch upon which the teeth are borne has become the most highly specialized of all the visceral arches. The massive upper pala- toquadrate (pterygoquadrate) segment {p-q., fig. 48) of this arch is as large as the lower mandil)ular segment or Meckel's cartila
The elasmobranch fishes elasmobranchfish03dani Year: 1934 THE ELASMOBRANCIl FISHES 45 the walls of the pharynx. In plan the branchial arches are essentially similar to the first two, but in the branchial arches there are typically four segments to every arch; these segments differ among themselves in minor details. The mandibular or first arch upon which the teeth are borne has become the most highly specialized of all the visceral arches. The massive upper pala- toquadrate (pterygoquadrate) segment {p-q., fig. 48) of this arch is as large as the lower mandil)ular segment or Meckel's cartilage (md.). This arch in Hepianchus, unlike that in most sharks, is closely bound to the cranium in two places, giving the amphistylic type of attachment (amphi, both; sfi'/ar,pillar). Anteriorly this union with the cranium is effected by the or])ital process of the quadrate coming in con- tact with the sides of the basal angle (,). Posteriorly the union is produced by a strong quadrate process (, fig. 48) joining the postorbital process of the cranium (). The upper and lower segments of the left side are connected in front with similar segments of the right side, but this articu- lation in Heptanchus is loosely made. Posteriorly the upper and lower segments are joined to each other by a simple double joint, and made fast medially by pronounced ligaments. The hyoid or second visceral arch is, as we have said, composed of two seg- ments. In Heptanch us these segments are slender and are entirely hidden in side view l)y the mandibular arch. The upper segment, unlike that in more highly specialized Elasmobranchs, is not a suspensorium for the mandibular arch; the lower segment is long and slender. Connecting the two ceratohyoids of opposite sides in the midventral line is an additional unpaired piece, the basihyal cartilage {bh., fig. 50a). Both segments of the hyoid are provided with numerous cartilaginous rays (, fig. 48) which support the main respiratory
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