288 REVIEWS 
Ostracoderms “as having definitely split off from the Elasmobranchs, 
from which they doubtless originally came.”” Dean believes in a wider 
separation, however, from the groups represented by recent forms ; but 
regarding the differences between Ostracoderms and Arthrodires, he 
makes the following significant remark: ‘“‘A renewed examination of 
the subject has caused me to incline strongly to the belief that Pterich- 
thys and Coccosteans are not as widely separated in phylogeny as Smith 
Woodward, for example, has maintained. But as far as present evi- 
dence goes, they appear to me certainly as distinct as fishes are from 
amphibia, or as reptiles are from birds or from mammals” (p. 24). The 
reference to Smith Woodward bears, of course, on the recognition of 
Arthrodires by that author as an order of Dipnoi. 
Whatever may be thought of the class Cycliae, there is no question 
but that Dr. Dean has scored an advance by elevating the Ostraco- 
derms and Arthrodires to a higher rank and placing them in close 
proximity to one another. A separation of the two classes is rendered 
necessary of course, thus prohibiting the revival of McCoy’s “ Placo- 
dermata,” by the absence of ‘‘jaws,”’ endoskeletal structures, and paired 
limbs in the first-named group. Nevertheless the two classes have a 
number of points in common, and should we be led to infer with Tra- 
quair an Elasmobranch derivation of the Ostracoderms, it would be 
natural to trace Arthrodires to the same source. Whether there were 
really “‘Agnatha,” and how far the archaic fishlike vertebrates were 
removed from the groups represented by living forms, must be left for 
future study to decide. Or possibly we may never have the solution of 
these perplexing problems. 
In one minor point only the reviewer finds himself in disagreement 
with Dr. Dean, and this relates to the subdivision of Arthrodires (or 
“Arthrognaths,” to use his new term) into Arthrodira proper and 
Anarthrodira. The latter includes Wacropetalichthys, Trachosteus, Mylos- 
Zoma, and certain transitional forms which the author promises shortly 
to describe. When the cranial and body armoring of Z7rachosteus and 
Mylostoma are made known, their position may become evident. At 
present we are acquainted only with the cranial osteology of Macropet- 
alichthys,and this is so far different from that of typical Arthrodires 
that in the reviewer’s opinion it cannot be retained in the same class. 
As typical of an independent family, it had best be removed with the 
Asterosteidae to a position amongst the Ostracoderms, as we certainly do 
not wish to make of it an independent class. The comparisons between 
