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 ( ft. o., fig. 47). Posteriorly 

 the union is produced by a 

 strong quadrate process (qd.p., 

 fig. 48) joining the postorbital 

 process of the cranium (po.o.) . 

 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 

 (h.r., fig. 48) which support the main respiratory structures. These rays on 

 the hyoid are considerably more complex than are similar cartilaginous rays 

 found on the branchial arches. 



The first branchial arch (fig. 49) may be taken as a type. Its segments, from 

 the dorsal to the ventral side, are : (1) the pharyngobranchial (pb.), (2) the 

 epibranchial (eb.), (3) the ceratobranchial (cb.), and (4) the small hypo- 

 branchial (hb.), respectively. These segments slant obliquely forward and 



Fig. 49. First brancliial arch, Heptanchus maculatus. 



li.r., cartilaginous branchial ray; cb., ceratobran- 

 chial; eh., epibranchial segment; ex.h., extrabranch- 

 ial cartilage; hb., hypobranchial ; pb., pharyngo- 

 branchial segment. 



