No.413.] REVIEWS OF RECENT LITERATURE. 419 
Of more general interest, however, are the author's views as to 
the relationships of the so-called * Arthrognathi" (= Arthrodira + 
Anarthrodira, the latter comprising only the genera Macropetal- 
ichthys and Holopetalichthys). Excluding them from both Pisces 
and Ostracoderms, Dean assigns them the rank of an independent 
class, which he conjectures may have been descended from primi- 
tive forms like Lanarkia in the Silurian. The prime reason for 
removing Arthrodires from Pisces rests upon the interpretation of 
their jaw-elements, which Dean regards as merely dermal ossifica- 
tions, showing **not the slightest evidence of their relation to endo- 
skeletal or gill-arch jaws." There is, however, strong presumptive 
evidence that a cartilaginous mandibular arch was present, the 
distal portion of which ensheathed the lower dental plate (** gna- 
thal Dean), and the proximal portion, or suspensorium, was 
attached to the head-shield.  How,.for instance, is one able to 
conceive of the attenuated mandibles of Titanichthys, except as 
imbedded in tissues corresponding to the Meckelian cartilage? 
The author advances some ingenious theories to explain the 
evolution of the articular joint between cranial and dorsal shields 
characteristic of Arthrodires, and, giving free rein to his imagina- 
tion, speculates on how such a joint might have arisen in the head- 
shield of Macropetalichthys. A slight impediment exists, however, 
in the way of accepting his conclusions, for the reason that we can- 
not admit any of his premises. Dean assumes, and so, too, have 
Newberry, Cope, and Eastman before him, for that matter, that a 
valid basis of comparison exists between this enigmatical form and 
various specified genera, where the facts prove such is not the case ; 
also that the arrangement of cranial plates and sensory canals in 
the genus under consideration is homologous with that pervading 
Arthrodires generally, which is an egregious error. 
The fact of the matter is that all writers on Macropetalichthys 
have been misled by deceptive appearances, perhaps to some extent 
also by preconceived ideas, and its osteology has not yet been 
correctly interpreted. Eastman’s and Dean's suggestion that there 
is a superficial system of investing plates arranged independently to 
a distinct substratum of bony elements is unsubstantiated, if not 
disproved ; and still more improbable is the notion that the “ head- 
Shield" is made up of combined cranial and dorsal shields. The 
transverse septum peculiar to this genus is an internal structure 
situated a long distance in advance of the posterior cranial border, 
and recalling in a measure the inwardly directed processes on the 
