rgS Arthrodires 



bility further exists that the "cirri" may turn out to be remnants 

 of cranial or facial structures of an entirely different nature. 

 In fact the very uncertain preservation of these parts renders 

 their evidence of little definite value. In but one specimen, 

 as far as I am aware, is there any evidence of the presence of 

 ventral cirri. 



The jaw parts in Palccospoiidylus are unknown. It is possi- 

 ble that the A'entral rim of the "nasal ring" may prove to be 

 the remains of the Meckehan cartilage (the cartilaginous core 

 of the lower jaw). 



It is possible that certain very faint ray-like markings noted 

 b}^ Professor Dean may be the basalia of paired fins. In such 

 case Palccospoiidylus can have no affinity with the lampreys. 

 Dr. Dean asserts that the presence of these, in view of the wide 

 dissimilarity in other and important structures, is sufficient 

 to remove Palaospondyliis from its provisional position among 

 the Cyclostcmes. The postoccipital plates may represent 

 a pectoral arch. It is, however, much more likely, as Dr. Tra- 

 quair has insisted, that the supposed rays are due to the reflection 

 of light from striations on the stone, and that the creature had 

 no pectoral limbs. 



The caudal fin, with its dichotomous rays, is essentially 

 like the tail of a lamprey. This condition is, however, found 

 in other groups of fishes, as among sharks and lung-fishes. 

 It is, moreover, doubtful whether the rays are really dichoto- 

 mous. 



It is possible that Palccospoiidylus may be, as Huxley sug- 

 gests, a larval Arthrodire. It is not probable that this is the 

 case, but, on the other hand, Palccospondylns seems to be an 

 immature form. According to Dr. Dean, it is more likely to 

 prove a larval Coccosteus, or the young of some other Arthrodire, 

 than a lamprey. Against this view must be urged the fact 

 that the tail of Palccospoiidylus is not heterocercal, a fact veri- 

 fied by Dr. Traquair on all of his many specimens. It is more 

 hke the tail of a lamprey than that of Coccosteus. It is, how- 

 ever, certain that it cannot be placed in the same class with 

 the living Cyclostonics, and that it is far more highly specialized 

 than any of them. In a still later paper (1904) Dr. Dean 



