SKULL — ELASMOBRANCHS 



part of the pharyngeal wall, two of which are connected with the styloid process, 

 the other (so-called hyoid) extends to the Ungual series. Besides these there are 

 vestigial cartilages at the openings of the gill-clefts and cesophageocutaneous 

 duct in Bdellostoma. The lingual cartilages are three, the anterior segment 

 consisting of two elements, the others being single. 



Attempts have been made to homologise the parts of the Cyclostome skull 

 with those of Gnathostomes, but, with the exception of the points made above, 

 few comparisons are convincing. The subocular bar has been compared with 

 the pterygoquadrate, but the branches of the fifth nerve pass 

 below it. The styloid process has more resemblance to the 

 hyoid. There is evidence that the branchial basket differs from 

 that of Gnathostomes as it is somatic rather than splanchnic 

 (p. 66 ) and in Petromyzon the ventral aorta lies within, rather 

 than ventral to the median ventral bar. It has also been 

 argued that the Ungual cartilages are really the lower jaw 

 structures of Gnathostomes and that the distinction between 

 the jawed \'ertebrates and Cyclostomes does not hold. But 

 the evidence for this is weak and needs considerable additional 

 support. 



PalcFospondylus. — At the anterior end of this problematic 

 fossil is a structure naturally regarded as a skull (fig. 85), but 

 concerning its parts there is much divergence of opinion. It 

 has recently been studied by means of sections, with the 

 following results: The cranial capsule is open above, below 

 and in front, and is connected on either side with the cavity 

 of the otic capsule which, as in Cyclostomes, has two semi- 

 circular canals. In front there are dorsal and ventral bars (rostralia), the 

 latter connected by a transverse bar (ampyx), behind which is a part of the 

 cranial floor (tauidion) and probably nasal capsules. Below there are four 

 branchial arches which are close, laterally, on either side to an angular bar 

 (gammation) in front of which is a pregammation. Gammation and pregamma- 

 tion are possibly hyoid and mandibular arches. Connected with the last 

 branchial arch on either side is a postbranchial plate. Sections show no traces 

 of ribs or appendages. 



Fig. 85. — Head 

 of PalcBospondy- 

 liis (Traquair). 

 M, nasal capsule; 

 o, otic capsule; 

 pp, postbran- 

 chial plates. 



ELASMOBRANCHII.— In all Elasmobranchs the cranium is a 

 continuous cartilage without separation into discrete parts, but with 

 large openings (fontanelles) and smaller foramina for nerves and 

 vessels. Except in Holocephali the visceral skeleton is connected 

 to the cranium only by ligaments or is articulated with it, the two 

 parts never fusing. In adults, especially of the larger species, it is 

 calcified on the outer surface, but bone is never developed. 



There are several separate parts in early development (fig. 86). 

 Notochord and parachordals, as elsewhere, form a basal plate extend- 

 ing laterally beneath the otic vesicles. The head is flexed just in 



