474 



CYCLOSTOMATA. 



Palceospondylus gunni. 



Under this title, Dr. Traquair has recently described a 

 remarkable fossil form from 

 the Old Red Sandstone of 

 Caithness. He speaks of it 

 as a "strange relic of early c ^?| 



vertebrate life." 



It is a dainty little creature, 

 somewhat tadpole-like at first 

 sight, usually under an inch in 

 length. The following char- 

 acters point strongly to its 

 affinities with Cyclostomata : 



(i.) u 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 capsules, 



(2.) "There is a median opening 

 or ring, surrounded with cirri, and 

 presumably nasal, in the front of the 

 head" (n., Fig. 153). 



(3.) " There are neither jaws nor 

 limbs." 



(4.) "The rays which support the 

 caudal fin expansion, apparently 

 springing from the neural and hoemal 

 arches, are dichotomised (at least 

 the neural ones), as are the corre- 

 sponding rods in the lamprey. " 



Just behind the head lie two small 

 oblong plates (.%., Fig. 153), closely 

 apposed to the commencement of 

 the vertebral column, one on each 

 side. The notochordal sheath is 

 calcified in the form of ring-shaped 

 or hollow vertebral centra with 

 neural arches. Towards the tail, 

 the arches are produced into slender neural spines, opposite which 

 are shorter haemal ones. 



FIG. 153. Restored skeleton 

 of Pakeospondylus. (After 

 TRAQUAIR.) 



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

 long lateral cirri ; v.c., cirri of ventral 

 margin ; ., nasal ring ; t.p., anterior 

 trabeculo-palatine part of cranium ; b., 

 anterior depression or fenestra ; c., 

 posterior depression or fenestra ; ., 

 lobe divided off from anterior part ; 

 p. a., posterior or parachordal part of 

 cranium ; x.> post occipital plates. 



