The Anatomy of Chlamydoselachus 425 



hm., pq.). In my four specimens these openings are very much aHke. They measure about 

 20 to 25 mm. long and are about 12 mm. wide when the pharynx is fully expanded. Thus 

 each internal spiracular aperture (i.s.c.) is large enough to admit a small finger. Its 

 posteromedial and anterolateral margins are well defined; they converge toward the 

 cranium and, when the pharynx is expanded, have the form of a furcula or "wishbone." 

 The posteromedial margin is formed by a prominent ridge where a fold of the mucous 

 membrane overlies a ligament (ligamentum postspiraculare inferior) extending along the 

 ventral surface of the hyomandibular cartilage and connecting it with the cranium. The 

 anterolateral margin is formed by a valvc'like fold or flap of the mucous membrane. 

 There is no very definite ventrolateral margin, for here the inner surface of the pharynx 

 slopes gradually into the spiracular cavity. This side lies toward the palatoquadrate. 

 When the pharynx contracts, the posteromedial and anterolateral margins of the internal 

 spiracular aperture approximate until the opening is reduced to a mere sht compressed 

 between the hyomandibular and palatoquadrate cartilages. No doubt the opening may 

 be completely closed by the contraction of the pharynx, but this can occur only after 

 most of the water has been expelled from the pharynx. 



Each internal spiracular aperture leads into a broad cavity or sac, the internal 

 spiracular cavity (Text'figure 82, i.s.c), which is no wider than its internal opening and 

 is about 7 mm. deep in its deepest portion. The roof of this cavity lies in close proximity 

 to the integument. By palpation I found that the overlying plate of tissues, covering 

 not only the deeper portion of the cavity but also its sloping side toward the palatO' 

 quadrate (Text'figure 82, p.q.), is decidedly thin. Evidently, it comprises Httle more than 

 integument and mucous membrane which come almost into apposition. In its structure 

 and in some of its relations this plate or membrane bears considerable resemblance to 

 the tympanic membrane of an amphibian. However, this membrane is evidently not 

 homologous with the structure described by Howes (1883) as the tympanic membrane 

 in Raja. Forming the anteromedial end of the internal spiracular cavity, beneath a flap 

 of mucous membrane, there is a pocket or caecum (c.l) which extends alongside the 

 hyomandibular in an anteromedial direction for a distance of about 10 mm. Its distal 

 end usually comes into contact with the auditory capsule of the cranium — a relation 

 which is most interesting when we compare the internal spiracular cavity of Chlamy^ 

 doselachus with the tympanic cavity of higher vertebrates. In three instances, I found 

 in this caecum a large gelatinous mass, almost cartilaginous in consistency, which was 

 easily removed. 



Before proceeding with the further description of the spiracle in my specimens 

 I quote the following from Goodey (1910.1, p. 550), who appears to be the only author 

 who has given any special attention to the spiracles of Chlamydoselachus: 



On removing the skin [of Chlamydoselachus] and carefully dissecting away the under' 

 lying spongy cutis which covers the jaw muscles, it is seen that the lumen of the spiracle 

 passes down into the oral cavity between the hyomandibular and the mandibular [sic] 

 cartilages. Just inside the external opening, the cavity becomes enlarged and a short caecal 



