56 ILLINOIS BIOLOGICAL MONOGRAPHS [56 



now penetrated the cartilage, which in the adult is entirely ossified in the 

 anterior region. A transverse section through this region of the 60 mm. stage 

 shows the beginning of the invasion of the cartilage by the bone, both dorsally 

 and ventrally. The posterior margin of the intemasal septum per- 

 sists as cartilage (Fig. 7). It extends dorsally on the ventral surface of the 

 dermal ossification as far as the interdigitation of this with the frontals. Ven- 

 trally it extends as the floor of the cavum cranii as far posteriorly as the orbito- 

 sphenoid (OS.). This is opposed to McMurrich's statement that the anterior 

 end of the ethmoid plate of the larva rem.ains unossified in the adult. 



Anterior to the olfactory foramen, the supraethmoid and the ectethmoids 

 of each side interdigitate in the wall of the nasal fossa. Comparison with the 

 condition of this region in the 32 mm. larva (Fig. 3), shows that the ossifica- 

 tions of the supraethmoid and the ectethmoid are perichondrial, and are devel- 

 oped on the outer wall of the massive intemasal septum. The jagged suture 

 between the two bones extends dorso-posteriorly as far as the frontal-supraeth- 

 moid suture (Fig. 10) and ventrally as far as the vomerosupraethmoid suture 

 (Fig. 6). The ectethmoids are in contact with the edges of the ventral surface 

 of the supraethmoid and curve posteriorly from it on the surface of the ethmoid 

 cartilage (Fig. 16). McMurrich (1884) says that the supraethmoid interdigi- 

 tates posteriorly with the orbitosphenoid, and, as my figures show, this is 

 effected by posteriorly extending spicules of the perichondrial ossification on 

 the ventral surface of the ethmoid cartilage, as the main parts of the two bones 

 under consideration, are widely separate (Fig. 16). 



Comparison of the median section of this region of Amiurus, with that of 

 Amia, (Sagemehl, 1884), shows several important differences. The solid cartilage 

 characteristic of the ganoid intemasal region is present in Amia, and there are no 

 ossifications in it. The ethmoid is a dermal ossification and lies near the dorsal 

 anterior end of the massive intemasal septum. The cartilage beneath this 

 ossification continues posteriorly as the solid tegmen cranii, whereas in Amiu- 

 rus, the posterior margin of the intemasal cartilage ends dorsally at the anterior 

 end of the frontals. There is no perichondrial ossification in this region in 

 Amia. The premaxillary bone abuts against the ventral surface of the anterior 

 end of the cartilage, there being no intermediate ossification, such as is found in 

 Amiurus; nor does the vom.er come in contact with the ethmoid, but, since the 

 ethmoid of Amia is comparable to the dorsal part of the dermo-supraethmoid 

 of Amiurus, this condition is not remarkable. The ossifications surrounding 

 the ethmoid plate of Amiurus have invaded the cartilage, while all the ossifica- 

 tions in this region of Amia are dermal. That the cartilage has not entirely 

 ossified in Amiurus is e\'idence that it has advanced but little farther in its 

 osseous development than has Amia. 



In the Characinidae (Sagemehl, 1885) — another of the lower teleost fami- 

 lies, which in American piscine classification (Gregory, 1907), is closely allied 

 to the siluroids as an offshoot from the lower branches of the teleost stock — 



