The Skull of Notothalamus Torosus 
Sarah Marimon 
There are twenty-eight bones in the entire skull of Notothal- 
amus torosus. There are, however, only fifteen different kinds, 
since thirteen are paired. 
These paired bones are: nasals, ectethnoids, maxillaries, fron- 
tals, parietals, squamosals, quadrates, ptergoids, occipitals, squamo- 
palatines, sphenethnoids, and (of the mandible) dentaries and artic- 
ulaires. 
The unpaired bones are: the parasphenoid and the premax- 
illary. 
The premaxillary (1) is the bone which forms the external 
division between the two nares. It consists of a rather thin 
dividing bone, which broadens out to form a broad flat base. Dor- 
sally the dividing bone diverges posteriorally to form two slender 
processes which join at the ends with the premaxillary processes of 
the frontal bones, and articulate on the exterior sides with the two 
nasal bones. Ventrally the dividing bone broadens out suddenly 
into a broad flat base which forms the most forward portion of the 
roof of the mouth, and serves as a connection between the two 
maxillaries. 
The nasals (2) are two irregularly shaped bones, each of which 
articulates on the interior side with a premaxillary process, on the 
exterior side of the ectethnoid and the maxillary. Anteriorally 
_ the nasals bound the dorsal side of the nasal cavity. 
The ectethnoids (3) are two triangular bones located on either 
FIGURES 
In lettering these figures I have used the following method: 
Each bone on the skull is marked with a large number. Throughout the 
figures, each bone goes by its number. When an articulation with a certain 
bone is indicated a small figure is used. The bone itself bears a large figure. 
When the bone borders on a cavity, or has a free edge, that portion of the 
bone is not numbered. 
Upper or outer surfaces are indicated by numbers, under or inner surfaces 
are indicated by the same numbers prime (1). 
