354 Bashford Dean Memorial Volume 



that seem to give any appreciable freedom of movement; but one should remember that 

 even a solid rod of cartilage is flexible. 



In most selachians, when the mouth is closed the hyomandibular is directed down' 

 ward, outward or even forward; but in Chlamydoselachus it is directed posteriorly. As the 

 mouth opens, its angles spread apart so that the entire oropharyngeal cavity broadens; 

 this is made possible by the length and mobility of the hyomandibular. When the mouth 

 is closed, the hyomandibular is neatly folded between the palatoquadrate and the vertebral 

 column, its anterior end lying somewhat apart from the cranium and a little above the 

 level of the anterior end of the dorsal border of the hyomandibular facet (af in Figure 3, 

 plate I). This facet is a broad groove extending longitudinally for a considerable distance 

 on the posterior part of the lateral surface of the cranium. When the jaws are opened, the 

 anterior end of the hyomandibular must slide posteriorly along the facet, while the 

 posterior end swings laterad and somewhat ventrad through an angle of about 45° 

 (Carman, 1885.2). Thus the articulation of the hyomandibular with the cranium is 

 a sliding joint of unusually loose construction, aiding greatly in the range of movement 

 of the hyomandibular. This peculiar hyostylism of the skull, together with the nearly 

 terminal position of the mouth, the long jaws and indeed the entire complex of adjust- 

 ments that gives Chlamydoselachus its enormous gape, are to be viewed as comparatively 

 recent adaptations of a highly specialised character. Goodey (1910.1, p. 550) says of the 

 jaws of Chlamydoselachus that "their disposition relative to the cranium is quite different 

 from that found in any Selachian whose skull I have been able to examine or see a figure 

 of. It resembles nothing among the Vertebrates so much, perhaps, as the general dispo- 

 sition of the jaws in certain of the Ophidia." 



Allis has described a palatal process of the palatoquadrate which serves as a support 

 for the soft parts of the horiziontal palatine shelf. ''The palatine process of Chlamy- 

 doselachus ... is a curved flat plate of cartilage, of nearly even width, that projects antero- 

 mesially beneath the anterior end of the neurocranium" (Allis, 1914, p. 354). The 

 hori2,ontal palatine shelf, which is evidently a homologue of the maxillary breathing valve 

 of certain teleosts, is fully described by Gudger and Smith (1933, p. 269). 



The cartilaginous lateral wall of the suprapalatine recess is perforated, on either 

 side, by the nasal fontanelle {naf. Figure 2, plate I). In its position and relations the 

 nasal fontanelle is, apparently, the strict topographical homologue of the fenestra choanalis 

 of Amphibia (Allis, 1913 and 1914). In its natural state the nasal fontanelle of Chlamy 

 doselachus is closed by a tough membrane (Allis, 1923, p. 132) which appears to be a part 

 of the cranium. This membrane is distinct from the mucous membranes lining the nasal 

 capsule and the mouth. The membrane evidently represents unchondrified portions of 

 the subnasal plate and the nasal capsule. "The nasal cavity of Chlamydoselachus is 

 thus separated from the suprapalatine recess by membranous and mucous tissues only, 

 and if these tissues were to be secondarily [sic] perforated ... an internal nasal aper- 

 ture would be formed which would lie directly above the horizontal palatine shelf" 

 (Allis, 1914, p. 355). 



