91 THE SKULL OF AMIURUS— KINDRED 9 



lie in this trough and are roofed by membrane which extends between the two 

 lateral projections and is attached dorsally to the integument. The olfactory 

 foramen, which is very large at this stage, occupies the anterior half of the 

 lateral wall of this region and the olfactory lobe protrudes through this fora- 

 men (Fig. 1). A short olfactory tract extends from the lateral part of the 

 lobe to the olfactory bulbus which hes just lateral to it. The ethmoid plate 

 extends laterally beyond these walls (Fig. 30) and forms the floor, called by 

 Gaupp (1906) the solimi nasi in Salmo, of the nasal fossa of each side (Fig. 2). 

 The anterior end of the plate projects as two massive cartilaginous processes, 

 the ethmoid cornua (Fig. 2). Just behind these a transverse ridge in the mid- 

 dle of the plate forms the anterior end of the cavimi cranii and marks the 

 anterior extent of the olfactory lobes. The ventral surface of the ethmoid 

 plate is slightly concave (Fig. 30). At the posterior end of the ethmoid region, 

 the anterior ends of the fused trabeculae and alisphenoid cartilages fuse with 

 the side walls and floor of the trough and the floor forms the anterior margin 

 of the fenestra hj'pophyseos (Fig. 2). Each lateral wall of the ethmoid region 

 is produced laterally into an ectethmoid process, which projects abruptly from 

 the external face of the cranial wall and the ventral face of which forms the 

 articular surface for the palatine cartilage (Fig. 1). The anterior face of the 

 process is concave near its ventral margin: the ophthalmic superficiahs ramus 

 of the trigeminus passes obUquely through the dorsal margin. The posterior 

 face of the process is fused medially to the anterior dorsal margin of the alis- 

 phenoid cartilage. The ventral part of this fused region is separated from the 

 trabecula by a small foramen, behind the ectethmoid process, the orbital fora- 

 men (Fig. 1). This foramen is the posterior Umit of the ethmoid region or 

 wall. As yet there is no internasal septum between the olfactory lobes of the 

 two sides. The obhque eye muscles have no relation to the ventral surface 

 of the ethmoid plate as they have in Salmo (Gaupp, 1906), but are attached 

 to the ventral margin of the orbital foramen posterior to the ectethmoid 

 process. 



Sagemehl (1885) discussed the morphology and development of the olfac- 

 tory region in the different famiUes of teleosts. He recognized three different 

 degrees of relationship between the olfactory organ, the bulbus olfactorius, and 

 the brain. In the first and most primitive relation which he described as the 

 Cyclostome type, the bulbus lies between the brain and the organ, within the 

 cavum cranii, closely fused to each. With subsequent development a long 

 tractus olfactorius is spun out between them, and the bulbus remains applied 

 to the olfactory organ. He called this the Selachian type, as it is most conunon 

 in this group. It is also found in the Cyprinoids, Siluroids, Mormyrids, and 

 Gadids among the teleosts. In these families the tractus always lies within a 

 canal directly continuous with the cavum cranii. 



In other teleost families, as represented by Salmo, a membranous inter- 

 orbital septum is developed between the orbits, on the dorsal surface of the 



