592 Arthrodires 



part shows a groove the edges of which are elevated, while the 

 surface on each side shows two depressions, like fenestrae, though 

 perhaps they are not completely perforated, and also a groove 

 partially divided off, posteriorly and externally, a small lobe. 

 In front there is a ring-like opening . . . surrounded by small 

 pointed cirri, four ventrally, at least five dorsally, and two long 

 lateral ones which seem to arise inside the margin of the ring 

 instead of from its rim like the others. The posterior part of 

 the cranium is flattened, but the median groove is still ob- 

 servable. Connected with the posterior or occipital aspect of 

 the skull are two small narrow plates which lie closely along- 

 side the first half-dozen vertebrae. 



"The bodies of the vertebrae are hollow or ring-like, and 

 those immediately in front are separated from each other by 

 perceptible intervals; their surfaces are marked with a few 

 little longitudinal grooves, of which one is median. They are 

 provided with neural arches, which are at first short and quad- 

 rate, but towards the caudal extremity lengthen out into slender 

 neural spines, which form the dorsal expansion of a caudal fin, 

 while shorter haemal ones are also developed on the ventral 

 aspect." 



Dr. Traquair concludes that "there seems to be no escape 

 from the conclusion that the little creature must be classed 

 as a Marsipobranch." "If Palcsospondylus is not a Marsipo- 

 branch, it is quite impossible to refer it to any other existing 

 group of vertebrates." 



Gill on Palaeospondylus. - - In 1896 Dr. Gill proposed to 

 regard Palceospondylus provisionally as the type of a distinct 

 order of Cyclostomes to be called Cyclia (KVK\OS, circle), from 

 the median ring on the head, whether nostril or mouth. Dr. 

 Gill observes: 



"Assuming the correctness of Dr. Traquair's description and 

 figures, we certainly have a remarkable combination of char- 

 acters. On the one hand, if the 'median opening or rim' is 

 indeed nasal, the animal certainly cannot be referred to the 

 class of Selachians or of Teleostomes. On the other hand, the 

 cranium and the segmental vertebral column indicate a more 

 advanced stage of development of the vertebrate line than that 

 from the living Marsipobranchs must have originated. We 



