96 ILLINOIS BIOLOGICAL MONOGRAPHS |96 



SUMMARY 



1. The chondrocranium of Amiurus is platy basic. 



2. In the chondrocranium of the 10 mm. Amiurus there is no intemasal 

 septum; the epiphysial bar is the only part of the chondrocranium dorsal to the 

 brain; it divides the opening in the roof of the primitive skull into an anterior 

 and a posterior fontanelle. 



3. The olfactory foramen lies in a sagittal plane and is very large in compari- 

 son with the olfactory tract. A solum nasi is present. The ectethmoid proc- 

 ess, a short transverse projection between the nasal fossa and the orbit, is 

 perforated by the ophthalmicus superficialis trigemini. An orbital foramen 

 is present posterior to the ectethmoid process. 



4. Trabecular and alisphenoid cartilages form the margins of a large fenestra 

 in the lateral wall of the cranium for the passage of the optic, oculomotor, tri- 

 geminal, abducens, and facialis nerves. The fenestra is closed by membrane 

 at the 10 mm. stage, except where the nerves pass. 



5. The alisphenoid cartilage is the homologue of the ala magna of the mam- 

 malian chondrocranium. 



6. The cavimi labyrinthii opens directly into the cavum cranii; there are 

 three septa semicircularia; no basicapsular fenestra is present; the glossopha- 

 ryngeal and vagus nerves issue from the cranium between the otic capsule and 

 the parachordal plate. The otic capsules are fused posteriorly with the occipi- 

 tal arch; a synotic tectum is lacking n the 10 mm. Amiurus. The hyomandibu- 

 lar articular surface lies external to the lateral semicircular canal. 



7. The parachordal cartilages lie lateral to the intercranial notochord, are. 

 fused anteriorly with the trabeculae, dorso-laterally with the otic capsules, and 

 posteriorly with the occipital arch. The notochord forms the axis of the poste- 

 rior part of the solid parachordal plate. The sacculi lie in recesses on the dorsal 

 surface of the parachordal plate on each side of the notochod. 



8. The ventral ends of the occipital arch are fused with the parachordals 

 posterior to the vagus foramen; the posterior end of the arch is inserted into the 

 anterior end of the third neural arch The hypoglossus nerve has dorsal and 

 vent'al roots united within the vertebral canal and a single lateralis ramus 

 issuing between the occipital arch and the anterior process of the scaphium. 

 The nerves posterior to the hypoglossus have the rami characteristic of true 

 spinal nerves and alternate with the neural arches. Two pairs of myotomes 

 are present lateral to the occipital arch, but their innervation by the nerves in 



