430 Bashford Dean Memorial Volume 



a pocket or caecum. This, like the pit or depression mentioned by Ridewood, is evidently 

 a vestige of the ventral end of a primitive gill'cleft. Although Ridewood was careful to 

 describe the relations of the pit or depression studied by him, he does not give any descrip' 

 tion of the pit itself further than that implied in the terms used. I infer that the pit or 

 depression examined by Ridewood is so simple that it does not need any further descrip' 

 tion. In Chlamydoselachus the opening is in series with the ventral ends of the branchial 

 clefts. In my four specimens it is from 8 to 15 mm. long and is bordered on the lateral 

 side (toward the mandible) by a crescentic valve4ike flap or fold of the mucous membrane. 

 The medial side has no definite boundary. The opening leads into a shallow cavity or 

 caecum (Text-figure 84, c.2) extending beneath the flap posteriorly and laterally for 

 a distance of from 3 to 5 mm., anteriorly for a distance of from 5 to 20 mm. Its average 

 extension anteriorly is about 12 mm., as shown in the figure. The structure and relations 

 of this cavity leave no doubt that it is a persistent ventral portion of a primitive gill' 

 cleft originally continuous with the dorsal portion now represented by the spiracle. 

 This primitive gill-cleft was bordered on the anterior side by the elements comprising 

 the jaw-cartilages, on the posterior side by the hyoid arch represented by the ceratohyoid 

 and the hyomandibular cartilages. 



Since writing the preceding paragraph and preparing the accompanying illustrations, 

 (Text-figures 82 and 84), I have found in the midst of a description by AUis (1916, pp. 

 110-111) of the mandibular artery of Chlamydoselachus, the following account of a some- 

 what similar pocket in the lining of the oropharyngeal cavity of his specimen : 



This latter branch [of the arteria mandibularis], on both sides of the head of this speci- 

 men, passes immediately anterior to a relatively deep tubular pocket, or recess, of the lining 

 membrane of the mouth cavity which, beginning slightly posterior to the angle of the gape, 

 extends dorsoposteriorly toward the quadrato-mandibular articulation. This pocket lies 

 along the external surface of the hind end of the palatoquadrate, between that cartilage and 

 those fibers of the musculus adductor mandibulae that pass uninterruptedly from the upper 

 to the lower jaw. Posteriorly it ends blindly, its blind end being attached to ligamentous 

 tissues which, continuing on in the line prolonged of the pocket, are attached to the hind 

 (distal) end of the palatoquadrate. The pocket thus lies morphologically anterior to the 

 palatoquadrate, in the relation to that cartilage that a persisting remnant either of the mandib- 

 ular cleft or of a premandibular cleft would have, and its position, posterior to the musculus 

 mandibulae, is not unfavorable to its being a remnant of either of those clefts, for the adductor 

 muscle, if it be derived from the superficial constrictor of the mandibular arch, could readily, 

 when it slipped from the external (actually posterior) edge of the arch on to its anterior 

 (actually lateral) surface, have acquired a position superficial, and hence morphologically 

 anterior, to the pocket. A branch of the artery is sent posteriorly, on either side of the 

 pocket, to the adductor muscle. 



It is evident, upon comparing this description with Text-figure 84, that the pocket 

 described by AUis does not have the same anatomical relations as the one described and 

 figured by me. 



