40 ILLINOIS BIOLOGICAL MONOGRAPHS [40 



hence the name proposed a priori by Kallmann (1837), the 'suprasphenoid, ' 

 may be applied to it, although it is to be understood that his criteria for dis- 

 tinguishing it are not used as the basis of the terminology, but rather its 

 independence as a connective tissue ossification above the parasphenoid and 

 between the ventral ends of the ahsphenoids. 



Schleip (1903) says that in Salmo the bone arises from a direct ossification 

 of the membranous wall at the anterior end of the eye-muscle canal, and that 

 it is indirectly connected to the parasphenoid and the ahsphenoids by mem- 

 brane which later ossifies. He recognizes it as part of the elements of the 

 primordial cranium which has no cartilage stage and says, p. 378; 



"Das Bindegewebe, aus dessen Ossifikation sie hervorgehen, geht in Telle 

 des Knorpelschadels iiber, verschliesst also lucken, die der Knorpelschadel 

 aufweist, und ist daher wohl nicht zu weit gegangen, wenn man es als Anlage 

 des Primordiacranium gehorig auflfast, als ein Teil desselben, der nicht zur 

 Verknorpelung kommt. " 



Even assuming this view to be true, the morphological relations of the bone 

 must be considered before it can be called the basi- or presphenoid. 



The parasphenoid of Amiurus in its development is like the large para- 

 sphenoid of the Urodeles which develops between the trabeculae and forms 

 the floor of the cranium in this group. Its lateral edges form the ventral 

 margin of the lateral wall of the orbital region as in Amiurus. It is related 

 to the orbitosphenoid in the anterior region, just as the parasphenoid of Amiu- 

 rus is related to the ossification around the anterior ends of the trabeculae. In 

 the 10 mm. stage of Amiurus, the place occupied by the parasphenoid of the 

 32 mm. stage is closed by a membrane which is separated from the trabeculae 

 and already shows signs of extending above and below them. In Amia, the 

 median floor of the cranium is continuous cartilage, a condition arising from 

 growth medially of the trabeculae, and the whole hypophysial fenestra is 

 closed, except for a small part just below the hypophysis, which is closed by the 

 parasphenoid. The parasphenoid here has the same superficial extent on the 

 ventral surface that it has in Amiurus, but its growth dorsally is restricted by 

 the cartilaginous cranial base. The orbitosphenoids of Amia, one on either 

 side of the cranium, are situated around the anterior margin of the optic 

 foramina, but do not have the same ventral extent as ihe ossification around 

 the anterior ends of the trabeculae of Amiurus. 



The parachordals. In the 10 mm. larva of Amiurus, the parachordal plates 

 posterior to the hypophysis, were rather widely separated from each other 

 (Fig. 2). Laterally they were continuous with the cartilage of the anterior 

 end of the otic capsules, this cartilaginous connexion forming the posterior 

 margin of the trigemino-facial foramen, (Fig. 2). In the 32 mm. larva this is 

 replaced by a thin lamella of perichondrial bone (Fig. 4). It appears as if 

 this condition arose from the earlier one by a growth of the otic capsule and the 



