THE ELASMOBRANCH FISHES 65 



The most posterior pliaryiigobrancliial, as a usual tiling', in l)oth sharks and 

 rays is fused with the one preceding, hence it may lose much of its character- 

 istic shape (Heterodontus, fig. 72). 



The epi- and ceratobranchials are the ray-bearing segments of the arches. 

 In all, except the most posterior arches, these segments are similar. In the last 

 arch, both of these segments are more or less modified. Generally this modifica- 

 tion takes the form of a thickening of the ceratobranchial (sharks) and an 

 atrophy of the epibranchial because of its fusion with its pharyngobranchial 

 cartilage. 



The hypobranchials, although perhaps more regular in a type like Chlamy- 

 doselachus (fig. 73a) than in Heptanchus, are generally more variable than 

 are any of the other segments. In the sharks the first, if present, is usually 

 small and is located between the distal end of the first ceratobranchial and the 

 hyoid cartilage (Heterodontus, fig. 73b, hb.^; Laemargus) . The second may 

 fuse with a similar one from the opposite side across the midventral line 

 {Scymnus, Laemargus) ; or the two may join a median unpaired cartilage, 

 the second basibranchial, as in Acanthias, Trygon, Heterodontus (hh., fig. 

 73b) ; or they may arch backward to join the enlarged median piece (mp.) as 

 in Torpedo (fig. 63) and Rhinohatis. Generally the third and fourth hypo- 

 branchials of pentanchid forms, except in some of the rays, are well developed 

 and are attached to the large median unpaired piece. In general, except Hep- 

 tanchus (fig. 50) , hypobranchials on the most posterior arch are lacking or are 

 fused with the unpaired median piece to which the third and fourth hypo- 

 branchials are attached. In the rays these segments may be present as plate- 

 like cartilages attached to the unpaired median piece (Raia erinacea, fig. 74b) . 



In the midventral line unpaired basal elements join the arches of the right 

 and left sides. The element connecting the two halves of the hyoid arch is 

 the basihyoid (bh., fig. 73) and those connecting the branchials are the basi- 

 l)ranchial cartilages (bb.). The basihyal cartilage may be a broad plate, per- 

 forated by the thyroid foramen {Chlamydoselachus, fig. 73a, bh.) ; or it may 

 bear an anterior glossal projection as a support for the tongue (Heterodontus, 

 fig. 73b, g.p.; ScylUum) . Again it may be a narrow band as in Acanthias or as 

 in Raia erinacea (fig. 74b) ; or it may be incomplete as in Torpedo (fig. 63). 

 Basibranchials are present as distinct irregular j)ieces of cartilage anteriorly, 

 but posteriorly they may fuse into a single mass. Generalized forms are char- 

 acterized by numerous basal elements. The first of these may be a peg-like 

 structure attached to the basihyoid (Chlamydoselachus) , or it may lie free be- 

 hind the basihyoid (Laemargus) . The second basibranchial is usually free and 

 the third, when present, is often attached to the larger posterior median piece 

 which may or may not be segmented. In the rays only the posterior median 

 piece is present (Rhinobatis, and Torpedo, fig. 63) . 



In the embryo of some forms well marked rudiments of still other branchial 

 arches persist, as we have seen in Heptanchus. Such rudiments are also pres- 

 ent in Chlamydoselachus, where a seventh arch has been described, and in 

 Heterodontus and in some of the rays, where a sixth arch may occasionally be 



