356 Bashford Dean Memorial Volume 



A rear view of the skull (Figure 6, plate II) shows the foramen magnum (/m), and 

 beneath it a small perforation, not labeled, for the extension of the notochord forward 

 into the basis cranii. The figure shows also a posterior view of the hyomandibular 

 articular facet (af), the postorbital process (pop), the ectethmoidal process (ecp), and the 

 eyestalk (es). 



As in other elasmobranchs, the brain does not fill the cranial cavity, which is shown 

 from the dorsal aspect in Figure 7, plate III. This figure shows also the olfactory capsules, 

 partly dissected, lying on each side of the broad rostrum. 



THE VISCERAL SKELETON 



In most elasmobranchs there are seven visceral arches: the mandibular, the hyoid, 

 and five branchial arches. In Chlamydoselachus and the notidanids there are additional 

 branchial arches making a total of eight visceral arches in Chlamydoselachus and Hexaw 

 chus, and nine in Heptanchus. 



Since the mandibular arch and the hyoid arch are usually regarded as derivatives 

 of primitive branchial arches, some embryologists use the term branchial arch for each 

 member of the entire series of visceral arches, and number them consecutively. In com' 

 parative anatomy it is more common to designate the mandibular arch and the hyoid arch 

 as such, and restrict the name branchial arch to the succeeding arches, which are numbered 

 separately. Thus, the third visceral arch is the first branchial arch. 



In Chlamydoselachus, as in other elasmobranchs, the mandibular arch (Figure 5, 

 plate II) is divided into an upper palatoquadrate or pterygoquadrate segment, and a lower 

 mandibular segment (Meckel's cartilage). The articulation between these two elements 

 is of a simple type, figured by AUis (1923) in his PI. XII. The ligaments connecting the 

 palatoquadrate with the mandible, and the mandibular arch with the hyoid arch, are 

 shown by AUis (1923) in his Pis. X and XI. AUis (1923, p. 149) states that the orbital 

 process of the palatoquadrate has a capsular sheath, and (pp. 208 and 209) refers to 

 a "somewhat ligamentous portion of the connective tissue that attaches the capsular 

 sheath to the anterior wall of the orbit." Garman (1885.2, p. 10) writes: "Some of the 

 most prominent differences between Chlamydoselachus and the notidanids are to be seen 

 in the attachments and articulations of this cartilage [the palatoquadrate]." 



As compared with the same structures in other sharks, the jaws of Chlamydo- 

 selachus (Te.xt'figure 24; Figure 5, plate II) are slender. This slenderness stands in marked 

 contrast with the condition found in Heptayichus (Daniel, 1934, Fig. 48), and is correlated 

 with a decided difference in the character of the teeth. In Chlamydoselachus, much 

 more than in Heptanchus, the jaws resemble branchial arches. 



The anterior labial cartilage (Figure 5, plate II) gives insertion to a long and stout 

 ligament attached to the cranium. From this ligament a series of ligamentous strings 

 are sent off to the upper lip. The posterior upper labial has no direct supporting relations 

 to the upper lip, but the posterior lower labial or mandibular labial gives attachment, at 

 its posterior end, to the tendon of the protractor anguli oris, and from its point of artic 



