ArT. 16, TWO SQUALODONTS FROM MARYLAND—KELLOGG. 47 
the mesethmoid which incompletely fills in the large frontal fontanelle 
and supports the nasals, leaving a small pair of passages on either 
side. They are here interpreted as transmitting the nasal branches 
of the ophthalmic (V) nerve. 
The dorsal surface of the skull is constituted largely by the 
maxillae and the premaxillae, and to a less extent by the frontals. 
The maxillae make up the greater part of the dorsal surface. They 
expand suddenly just posterior to the maxillary notches, forming 
what may be called the frontal plates. These partially roof over the 
temporal fossae. Each maxilla terminates slightly in advance of 
the supraoccipital posteriorly. The internal margin of the maxilla 
is In contact with the premaxilla for practically the entire length. 
The surface of the maxilla is slightly depressed opposite the nasals, but 
plane elsewhere. The free or external margin is thinner than the 
internal. The orbital plate of the maxilla does not completely cover 
the thick apophysis of the supraorbital process of the frontal but 
leaves a considerable portion of the latter exposed in front and at 
the side. The broad base of the rostrum is formed largely by the 
lateral expansion of the maxillae. 
The cranial region of the skull is quadrilateral in outline. As 
remarked above, the frontal plates of the maxillae constitute the 
greater part of the dorsal surface, the lateral extensions of the 
frontals being concealed for the most part by these bones. The nasal 
bones do not overhang the nasal openings and are considerably 
shortened. Posteriorly, they are suturally united with the frontals. 
The nasals present a quadrangular outline, truncated anteriorly, and 
emarginate posteriorly. 
The rostral or anterior wall of the brain cavity is composed largely 
of a vertical sheet of bone formed by the paired frontals. Between 
these is a large frontal fontanelle which as it closes is filled in by the 
mesethmoid. Support to this view is lent by the figures of Squalodon 
zutellv.” The fontanelle is still open in Zittel’s specimen. The 
element labeled pa. [= parietal] in his figure is really the supra- 
occipital. The Maryland squalodont and Zittel’s specimen admirably 
supplement each other and there is little chance for error in defining 
the relations of the various bones. Each frontal sends back a thin 
isthmus which unite by suture along the median line and form the 
vertex of the skull. Posteriorly, they are in contact with the supra- 
occipital and anteriorly are synostosed with the nasals. Laterally and 
at a lower level than the vertex the frontals widen out, support the 
maxillae, and give rise anteriorly to the supraorbital processes. The 
apophyses of the supraorbital processes are rather pointed, the 
7 Zittel, K. von, Palaeontographica, Stuttgart, vol. 24, pl. 35, figs. 1 and 3, 1876-77. 
