MAXILLARY AND VOMER BONES OF POLYPTERUS er 
The hyomandibula of the Palaeoniscidae is said by Traquair 
to be an elongated and slender bone which descends downward 
and backward from the squamosal region to the neighborhood 
of the quadrate articulation, thus apparently articulating with 
the neurocranium dorsal to the vena jugularis, as it does in all 
recent Teleostomi; this showing that these early Chondrostei 
had already departed markedly from the Plagiostoman type. 
In the Platysomidae, a family of the Chondrostei closely 
related to the Palaeoniscidae, the teeth were feeble or wholly 
wanting (Traquair, ’79). The feeding habits of these fishes 
could not then have required a fixed and rigid support for their 
teeth, and doubtless in correlation to this the maxillary is not 
fixed in position by attachment either to the palatoquadrate or 
to the suborbital and cheek bones. That it was not attached 
to the cheek bones is evident from the fact that its dorsal edge 
is no longer cut away to fit against those bones, as it is in the 
Palaeoniscidae, and in Platysomus it even overlaps them ex- 
ternally. There must then have been a more or less developed 
supralabial furrow in these fishes, and quite possibly it existed 
in the condition that I have described in 113-mm. embryos of 
Amia. These fishes thus presented conditions favorable to the 
development of a maxillary bone such as is found in Amia. 
In the Crossopterygii there are apparently two distinctly 
different types of maxillary, one resembling somewhat that found 
in the Palaeoniscidae and the other that found in the recent 
Polypterus. The former type is found in fishes of the suborder 
Rhipidistia of Smith Woodward’s classification, and the other in 
those of the suborder Actinistia. 
In the Rhipidistia there are, according to Smith Woodward 
(91), dentigerous premaxillaries which are usually fused with 
each other, and also, more or less, with small plates which intervene 
between them and the frontals. The maxillary is said to be 
_ dentigerous, and to be bounded above by the suborbital and 
cheek bones; and in restored figures of these fishes it is ap- 
parently shown in articular contact with the latter bones and 
frequently tuberculated. In restored figures of Rhizodopus, its 
hind end is shown overlapping externally to a considerable 
