The Anatomy of Chlamydoselachus 



429 



surface in some spiracles of my specimens, structures which may be vestiges of gill' 

 filaments. These structures are low ridges, soft when palpated but not disappearing 

 entirely when the mucous membrane is stretched at right angles to their long axes. They 

 are spaced regularly, like gill'filaments. In number, position, length and direction they 

 resemble the pseudobranchial filaments on the opposite side of the pseudobranchial 

 chamber, but they are usually broader and are never so high. I suspect that if fresh 

 specimens were available, the presence of vestigial gill'filaments on the posteromedial 

 wall of the pseudobranchial chamber could be conclusively demonstrated. 



A pit or depression representing the ventral end of a primitive gill'cleft extending 

 between the hyoid and mandibular arches has been described by Ridewood (1896) in 

 Galeus, Carcharias, Zygaena, Triads and Chiloscyllium. It is faintly marked in Mus- 

 telus, but is absent in Scyllium, ?{ptidanus and Acanthias. Concerning this pit or de' 

 pression Ridewood writes as follows: 



If a line be drawn joining the lower ends of the pharyngeal apertures of the branchial 

 clefts, it will pass through the lower or anterior extremity of the pit, just as a curved line 

 joining the upper ends of the branchial clefts will, if produced, pass through the inner or supe- 

 rior edge of the pharyngeal aperture of the spiracle. It is universally admitted that the 

 spiracle of sharks represents only the upper part of the hyoid cleft, the middle and lower 

 portions being obliterated. Hence, in this depression of the mucous membrane, is a structure 

 which, in complete absence of evidence to the contrary, may be regarded as the internal or 

 pharyngeal portion of the lower half of the hyoid cleft. 



In my four adult specimens of Chlamydoselachus I found, on each side of the floor 

 of the pharynx, between the ceratohyoid and mandibular cartilages and directly ventral 

 to the internal spiracular aperture, a large opening (Text-figure 84, v.g.c.) leading into 



Text-figure 84. 



Left internal spiracular aperture and vestigial 



gill-cleft (x 0.86) of Chlamydoselachus in their 



relation to each other and to the adjoining 



cartilages. 



hr.c.l, first gill-cleft, showing the demibranch attached to 

 the hyomandibular and ceratohyoid cartilages; br.c.2-3, 

 second and third gill-clefts; c.l, caecum of the internal 

 spiracular cavity; c.2, caecum of the vestigial gill-cleft; 

 ch, ceratohyoid cartilage; cr, cranium; i.s.c, internal 

 spiracular aperture and cavity; hm, hyomandibular 

 cartilage; l.p.i., ligamentum postspiraculare inferior; 

 m, mandible or Meckel's cartilage; pq, palatoquadrate; 

 s.c, spiracular canal; v.g.c, vestigial gill-cleft. 



Drawn from specimen No. I in the collection of the 

 American Museum of Natural History. 



