100 Bashford Dean Memorial Volume 



Cephalaspidomorphi and for the other the term Pteraspidomorphi. The knowledge which we 

 now possess concerning their phylogeny and relationships shows that the Petromyzontia and 

 Myxinoidea have degenerated in certain respects, as, for instance, with regard to the degree of 

 development of the skeleton and the absence at least of pectoral fins and shoulder-girdle, but 

 that otherwise on the whole they remain in a very lowly organized stage of evolution and that 

 they are primarily agnathous forms. 



Stensio believes that the Myxinoidea are "descendants either from Palaeospondylus 

 or from primitive Heterostraci." He suggests the following classification to express the 

 interrelationship of the Ostracodermi : 



Vertebrata Craniata 

 Branch I Agnathi 



Class Ostracodermi (Cyclostomata) 

 Sub class A Pteraspidomorphi 

 Order 1 Heterostraci 

 Order 2 Palaeospondyloidea 

 Order 3 Myxinoidea 

 Sub class B Cephalaspidomorphi 

 Order 1 Osteostraci 

 Order 2 Anaspida 

 Order 3 Petromyzontia 

 Branch II Gnathostomi. 



From the above brief resume of the literature it is evident that the investigators in 

 the two fields which Dean believed would yield the most informative evidence, namely 

 embryology and paleontology, agree that the Myxinoidea are primitive; but there is a 

 difference of opinion as- to whether these low vertebrates belong to the Gnathostomes 

 or to the Agnathostomes. 



SUMMARY 



A general summary of the entire paper is hardly necessary, as a discussion and re- 

 sume have been given within each section. The data which Doctor Dean left in the form 

 of notes, tables, graphs, and sixteen excellent drawings, which have formed the chief 

 basis of this paper, have made many important contributions to our knowledge of the 

 Myxinoidea. The information which he has supplied makes it inexcusable to continue 

 publishing in text'books the statement that some of the myxmoids are protandric herma- 

 phrodites. He has shown that Bdellostoma is certainly not hermaphroditic in any respect, 

 and has raised grave doubts as to whether Myxine is functionally hermaphroditic, even 

 though it is structurally. The extensive investigations of the Schreiners have proved that 

 Dean was correct in doubting that Myxme is functionally an hermaphrodite. 



Dean's data regarding the eggs are very interesting and instructive, and furnish us 

 with much information concerning phenomena related to the development of the ova, 

 their escape from the ovary, and the time of spawning. The following points seem almost, 

 if not quite established as facts: 



1. When some eggs have grown to a length of 2-5 mm., all the smaller eggs are 

 arrested in growth while a favored few continue to develop to maturity; the favored eggs 

 are approximately segmental, one to each myomere, and continue to develop to the condi- 



