56 PETR0MYZ0NT1A 



(Dohrn) to show that the Cyclostomes are degenerate fish, derived 

 from some member of the class Pisces. The facts we have just 

 cited sufficiently dispose of this view ; these characters, at all events 

 (hypophysial sac, fully developed anterior myotomes, larval thyroid 

 gland, rasping 'tongue,' etc.), cannot be due to . degeneration ! 

 Nevertheless, it can hardly be doubted that the Myxinoids, if not 

 the Petromyzontia as well, show some signs of degeneration. In 

 the former group the vestigial eyes, for instance, and perhaps the 

 absence of median fin muscles and many characters of the skeleton, 

 may safely be attributed to degeneration. The total absence of 

 paired limbs and girdles is, of course, a very important feature ; 

 there is no satisfactory evidence that the Cyclostomes ever had any, 

 in spite of Dohrn's suggestion that the small folds at the sides of 

 the cloaca represent vestiges of pelvic fins. Whether the ancestral 

 Craniate, from which both the Gnathostome and the Cyclostome 

 groups diverged, possessed biting jaws or not, is a cpiestion we can 

 scarcely hope to solve without palaeontological evidence (Howes 

 [221]). 



Incertae sedis. 



Family Palaeospondyudae. This family contains only the very 

 interesting extinct fishlike creature from the Middle Old Red Sandstone of 

 Scotland, named Palaeospondylus Gunni by Traquair, who first described 

 it and considered it to be possibly a fossil Cyclostome. Unfortunately 

 Palaeospondylus is very small, its skeleton is ill -preserved, and its 

 structure still very imperfectly understood, in spite of the careful 

 researches of Tra<piair [463], Dean [107], and Sollas [411]. 



The elongated body appears to have been naked ; no trace of teeth has 

 been found (Fig. 38). The skull is relatively large ; it has an extensive 

 brain-cavity with side walls, a continuous floor marked with an infundib- 

 ular depression, but probably an incomplete roof. Behind the large 

 orbits are the auditory capsules continuous with the cranial wall, and in 

 front are ill-defined structures which probably represent paired olfactory 

 capsules. From the extreme anterior end of the skull project some 

 eleven slender processes attached in a circle to a basal ring ; they 

 seem to represent tentacles surrounding a median aperture, which 

 might be the nostril or the hypophysis, but much more probably the 

 mouth. Below the anterior region of the skull is a T-shaped element 

 of doubtful nature, but probably belonging to the visceral skeleton, of 

 which distinct traces have been described by Sollas. About four 

 branchial arches can be made out, and in front of them indications 

 of an upper and lower jaw. Attached to the hindermost arch are a 

 pair of large plates projecting backwards behind the skull. The vertebral 

 column has a large number of ring-like centra with neural arches. The 

 tail is diphycercal, bears a caudal fin supported by median prolongations 

 of both the neural and the haemal arches, forming delicate rays some- 

 times branched, and strikingly like those of the Cyclostomes. There are 

 no ribs, nor have any certain traces of paired fins been found, though it 



