SKULL CYCTOSTOMES 



79 



Parts of the early skull of the Amnioccetes stage of Petromyzon (fig. 82) are 

 comparable with those of Gnathostomes. The notochord extends forwards to 

 the hypophysial region and is flanked by parachordal -plates which fuse later 

 with the otic capsules. A trabecula on either side 

 extends lateral to hypophysis and infundibulum, 

 the two trabeculas uniting in front in the 

 equivalent of an ethmoid plate, and enclosing a 

 fenestra hypophyseos. A slender bar passes 

 obliquely forwards and laterally from the para- 

 chordal, in front of the otic capsule. This has 

 been called both a quadrate and a hyoid, but is 

 certainly neither. 



The same parts are recognizable in the 

 skull of the Adult lamprey (fig. 83). 

 Parachordals and chorda form a basal 

 plate, limited laterally by the otic capsules 

 and in front by the fossa hypophyseos 

 which is bounded laterally by the trabeculas. 

 Farther forwards is a broad ethmoid plate 

 below the olfactory organ. The roof of the 



brain case is largely membranous, but there is a synotic tectum con- 

 necting the otic capsules of the two sides. The single median nasal 

 capsule is attached to the rest of the cranium by ligaments. 

 Beyond this, comparisons with other skulls are uncertain. 



Fig. 82. — Chondrocranium of 

 larval (Ammoccete) lamprey 

 (Schneider, '02). h, "hyoid;" 

 nc, notochord; oc, otic capsule; 

 pa, parachordal; tr, trabecula. 



Anterior to the ethmoid region is a 'posterior median dorsal cartilagen 

 flanked by 'lateral cartilages' and continued forwards by an 'anterior media,' 

 dorsal cartilage,' these supporting the buccal cavity in front of the median naris. 

 These cartilages support an annular 'labial cartilage' with a' styliform carti- 

 lage' on either side. Lateral to the trabecula is a subocular bar which has a part 

 terminating in a 'styloid process' and connected with the branchial basket. 

 This basket is composed of a connected and fenestrated cartilage surrounding the 

 gill region and enclosing also the pericardium. It consists of four longitudinal 

 bars (hypochordal, epitrematic, hypotrematic and ventral) connected by 

 vertical bars between each two gill-clefts. The so-called tongue contains a 

 series of large Hngual (basal) cartilages to which the muscles of this rasping 

 organ are attached. 



In the Myxinoid skull (fig. 84) parachordals, otic and nasal capsules, trabeculae 

 and subocular bar are easily made out, but other homologies are difficult. The 

 cranial roof is membranous, the median nasal capsule is fenestrated and the 

 nasal duct, leading back from the naris, is supported by a series of cartilage rings. 

 Four pairs of cartilages support the tentacles. The branchial skeleton is less 

 complete than in Petromyzontes, there being two or three bars in the anterior 



