PAL&OSPOND YL US. 



527 



Lampreys are distributed in the rivers and seas of north and south 

 temperate regions. They are often used as food. Besides Petromyzon 

 there are several related genera, e.g. Mordacia and Geotria, from the 

 coasts of Chili and Australia, and Ichthyomyzon, from the west coast 

 of N. America. Certain structures called " conodonts," from very 

 ancient (Silurian) strata, have been interpreted as teeth of lampreys 

 or hags. 



Palseospondylus gunni. Under 

 this title Dr. Traquair has de- 

 scribed a very remarkable fossil 

 form from the Old Red Sandstone 

 of Caithness. He speaks of it 

 as a " strange relic of early verte- 

 brate life." 



It is a dainty little creature, 

 somewhat tadpole-like at first 

 sight, usually under an inch in 

 length. The following characters 

 point strongly to its affinities with 

 Cyclostomata : 



(j> "The skull is apparently formed 

 of calcified cartilage, and devoid of 

 discrete ossifications." An anterior part 

 is comparable to the trabecular and 

 palatal region of a lamprey's skull ; 

 a posterior part is comparable to the 

 parachordal region and auditory cap- 

 sules. 



(2) "There is a median opening or 

 ring, surrounded with cirri, and presum- 

 ably nasal, in the front of the head" 

 (Fig. 278, .) 



(3) "There are neither jaws nor 

 limbs." 



(4) "The rays which support the 

 caudal fin expansion, apparently spring- 

 ing from the neural and haemal arches, 

 are dichotomised (at least the neural 

 ones), as are the corresponding rods in 

 the lamprey." 



Just behind the head lie two small 

 oblong plates (Fig. 278, x.), closely 

 apposed to the commencement of the 

 vertebral column, one on each side. 

 The notochotdal sheath is calcified in 

 the form of ring-shaped or hollow verte- 



35 



FIG. 278. Restored skeleton 

 of PalcEospondylus gunni. 

 After Traquair. 



d.c., Cirri of dorsal margin ; I.e., 

 long lateral cirri ; v.c., cirri of 

 ventral margin ; ., nasal ring ; 

 t.p.t anterior trabeculo-palatine 

 part of cranium; &, anterior 

 depression or fenestra ; c., pos- 

 terior depression or fenestra ; 

 a., lobe divided off from anterior 

 part ; p. a., posterior or para- 

 chordal part of cranium; x. 

 post-occipital plates. 



