340 Bashford Dean Memorial Volume 



the primitive stock from which elasmobranchs and teleostomes diverged. On this point. 

 Dr. W. K. Gregory, in a personal communication, commented as follows : '"''The idea that 

 Chlamydoselachus stands nearer to the true fishes than do the sharks proper, is without 

 a vestige of real evidence in its favor and with a mountain of evidence against it." 



In Chlamydoselachus the external openings of the spiracles (Text 'figures 70, p. 396; 

 and 124, p. 489) are very small. In the notidanids the spiracles are said to be small. 

 In some sharks that certainly bear no close resemblance to Chlamydoselachus, spiracles 

 are absent altogether. In skates and rays, which are bottom'dwelling forms, the spiracles 

 are proportionally large. It has been inferred that spiracles were developed in connection 

 with a sea-bottom habitat; but this is true only of the valvular apparatus which, in skates 

 and rays, enables the spiracle to function for the inta\e of water when the mouth is buried 

 in sand or mud. In Squatina, a bottom-d welling shark, the spiracles sometimes admit 

 water to the oropharyngeal cavity. But sharks are characteristically free-swimming 

 forms in which the spiracles, if present, serve merely for the exit of water from the pharyn- 

 geal cavity, thereby retaining their primitive function as gill-sHts. This is the function 

 of the spiracles even in Chlamydoselachus, as will appear from the description of the 

 spiracular canal (p. 423) in the section on the respiratory organs. 



The small si7;e of the external spiracular openings of Chlamydoselachus affords 

 evidence that the spiracles are in a vestigial, not an incipient condition. Spiracles have 

 not arisen de novo; they represent merely a modification, sometimes accompanied by 

 a change in function, of a primitive pair of gill-slits situated between the mandibular and 

 the hyoid arches. In the process of transformation of this primitive anterior pair of 

 gill-slits into spiracles, the ventral portions of the openings close, while the dorsal portions 

 persist — as is shown in Text-figure 62, p. 388. The internal aperture is much larger than 

 the external. If one opens the mouth of any shark possessing spiracles, he will find a pair 

 of large internal spiracular openings resembling gill-sHts, in exact serial relation with the 

 dorsal portions of the gill-slits. In Chlamydoselachus, whose external spiracular opening 

 is a slit only 2 or 3 mm. long (Text-figures 70, p. 396; and 124, p. 489), the internal spiracu- 

 lar orifice is an elliptical aperture more than 20 mm. long and wide enough to admit easily 

 the blunt end of a pencil. As in many other selachians, the spiracles of Chlamydoselachus 

 possess vestigial gills, called pseudobranchs. 



FINS, PAIRED AND UNPAIRED 



The bunching of the pelvic, ventral and dorsal fins near the caudal (Text-figure 1) 

 gives color to Carman's view (1884.1, .2) that these fins provide the creature with a ful- 

 crum from which to strike. This arrangement of the fins is a very special feature. The 

 pelvic fins, the anal fin and the ventral lobe of the caudal fin are sufficiently large to in- 

 dicate that Chlamydoselachus is not closely confined to the sea bottom. The shape of the 

 tail is much like that of Heptanchus (Text-figure 3). 



The weakness of the fins of Chlamydoselachus is due not only to the softness and 

 fineness of the dermal fin rays, which are exoskeletal structures, but also to the rudimen- 



