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THE AMERICAN NATURALIST [Vol. XL 



just posterior to the nasals and form a considerable part of the 

 roof of the skull. Like the nasals, these two bones, when taken 

 together, have somewhat the shape of an arrowhead, the tip of 

 the head again being towards the posterior. Along the middle 

 line, where the bones articulate with each other, is sometimes 

 seen a well marked ridge. Each frontal forms the posterior half 

 of the lateral border of the corresponding nasal opening, and 

 articulates laterally with the maxillary, prefrontal, and parietal; 

 anteriorly with the nasal, and medially with its fellow of the oppo- 

 site side. Ventrally the frontals are more or less closely united 

 with part of the cartilaginous cranium, to be described later. 



The prefrontals (Pf.) are two elongated bones in the roof of 

 the skull, on the antero-medial border of the orbits. Each bone 

 articulates anteriorly with the corresponding maxilla, medially 

 with the frontal, and posteriorly with the extreme anterior end 

 of the parietal. 



The parietals (P.) are two large bones that form the greater 

 part of the roof of the cranial cavity. The posterior half of each 

 bone is broad and angular, while the anterior half is long and nar- 

 row, and extends forward to articulate with the posterior end 

 of the prefrontal, as has already been stated. The posterior 

 halves of the two parietals articulate with each other, but the nar- 

 row anterior portions are separated from each other, and into 

 the space thus formed the posterior ends of the two frontals pro- 

 ject and articulate. The narrow, anterior part of the parietal 

 overlies and is more or less closely attached to the ethmoid, pres- 

 ently to be described. The broad, posterior part of the parietal 

 articulates laterally with the squamosal, and posteriorly with the 

 lateral occipital. The sagittal suture, between the two parietal 

 bones, extends back to the antero-dorsal border of the foramen 

 magnum. A very small portion of this l)order is formed by the 



um. The ossified portion of \\w ftlim;)i(l is shown at /•>'. in Figs. 

 1 and 3. It is an elongated n..l of l.onr. attached dorsally to the 

 parietal, as has been mentioned, and ventrally to the parasphenoid 



