574 SUMMARY OF CURRENT RESEARCHES RELATING TO 



entiated from a primitive cavity. Fiirbringer is of opinion that the 

 position of Myxinoids, and their relation to the lampreys and the other 

 Craniota, would be best expressed by a classification based on the number 

 of semicircular canals, but as yet is hardly prepared to formulate a new 

 classification. 



Position of Palseospondylus. — Dr. Bashford Dean * discusses the 

 literature of Palseospondylus and its discrepancies, the known and 

 questionable structures of the fossil, its developmental characters, and 

 views as to its probable kinships. He leads up to conclusions as to the 

 affinities and systematic position of the fossil and the arrangement of 

 lower Vertebrates. 



He thinks that the single feature in Palseospondylus which forcibly 

 suggests a Marsipobranch, is the anterior " unpaired narial opening " 

 with its surrounding " cirri." But it is not certain that the opening is 

 narial. 



Bashford Dean tabulates the evidence showing that the animal 

 undergoes a striking change in the form of its body and head, as it 

 increases in size. " From the condition in the earliest specimen the 

 head decreases steadily in size (both length and width) in proportion 

 as the length of the vertebral column increases. Thus the head, from 

 measuring in the earliest stage about fifty per cent, of the entire length 

 of the animal, becomes reduced to but fifteen per cent, of this length in 

 the largest specimen — a result which becomes convincing in view of the 

 transitional stages measured. The post-occipital plates are found to 

 decrease in size as the length of the animal increases ; their absence in 

 the largest specimen, however, by no means proves that these structures 

 have been normally lost, although it is remarkable that they have not 

 been preserved in a specimen whose anterior vertebral column^ is so 

 perfectly preserved as in the present case. The modification in the 

 vertebral column is a very striking one. From the earliest condition 

 the column decreases in relative thickness until the individual measures 

 24 mm. or thereabouts in length. In the largest specimen, however, its 

 proportionate thickness is half again as great as in the specimen of 

 24 mm." The author thinks it fair to conclude that Palseospondylus can 

 hardly be looked upon as having attained its definite form, and that it is 

 a larval form, perhaps of an Arthrodire, e.g. Coccosteus. 



Mr. J. Graham Kerr f points out some resemblances between this 

 form and a young Dipnoan, as suggested by a study of the Lepidosiren 

 larva. The post-occipital plates" of the former suggest the so-called 

 cranial ribs of the latter, and some resemblances between the skull of 

 Ceratodus and that of Palseospondylus are also indicated. It is to be 

 noted that Dipnoans were abundant about the times in which Palseo- 

 spondylus lived. A point apparently against the Dipnoan nature 

 of Palseospondylus lies in the highly developed vertebras, but this objec- 

 tion tells equally against its being a Cyclostome. But if the tubular 

 centra are really, as Dr. Traquair suggests, formed in the sheath of 

 the notochord, it is interesting to note that the Dipnoi are potentially 

 chordo-centrous, and the Cyclostomes essentially arco-centrous. 



* Mem. New York Acad. Sci., ii. (1899) pp. 1-31 (1 pi.). 

 t Proc. Cambridge Phil. Soc, x. (1900) pp. 29S-9. 



