58 ILLINOIS BIOLOGICAL MONOGRAPHS [58 



forms the floor of the nasal process and the basal part of the process. The 

 ossification of the dorsal surface unites with a descending perichondria! wall 

 from the ventral surface of the roof of the intemasal septum and forms the 

 olfactory canal. Thus, in the adult, there is no cartilaginous floor in the nasal 

 fossa, but it is formed by the premaxillary bone and tough connective tissue. 

 In spite of all the perichondria! ossification which has taken place in this region, 

 the articular surface for the palatine and a small strip ventral to it remain 

 uncovered (Fig. 16). This cartilage lies between dorsal and ventral ossifica- 

 tions of the ectethmoid anteriorly, but posterior to the articular surface these 

 parts unite (Fig. 16). The ventral ossification interdigitates anteriorly with 

 the vomer, and, at the very edge of the cranium, with the supraethmoid (Fig. 6), 

 Further contact between the ventral ossification of the ectethmoid and that of 

 the supraethmoid is prevented by a wide expanse of cartilage visible upon 

 removal of the vomer (Fig. 16). This figure also shows the distance between 

 the ectethmoids of the two sides and the very regular edge that each has. They 

 are also separated from the orbitosphenoid (OS.) by cartilage. 



Posteriorly and above, the dermal ossification attached to the margin of 

 the bone is indistinguishably fused with the perichondrial ossification surround- 

 ing the foramen orbito-nasale (Fig. 10). This dermal ossification is a very 

 pronounced process projecting at right angles from the cranial wall. Its 

 medial end is continuous with that part of the bone which, with the orbito- 

 sphenoid, forms the upper and lower margins of the orbital foramen (Fig. 20). 

 These are the main features that distinguish the bone. Its dorsal surface is 

 covered with small ridges which radiate from the center. There are also 

 numerous nerve foramina for twigs of the ophthalmicus superficialis trigemini, 

 but none of these are connected with a lateral line canal as McMurrich (1884b) 

 stated, because no such canal is included in the ectethmoid. The posterior 

 margin of the lateral process is continuous with a similar one on the margin of 

 the frontal of the same side. The inner surface of the bone is covered with a 

 thin layer of cartilage (Fig. 7). 



Cuvier (1828) called this bone the 'frontal anterieur' and described it 

 briefly in the perch. In his diagnosis, he said that it enclosed the olfactory 

 nerve, was not entirely ossified, and had an articular surface for the palatine 

 and maxillary bones. The suborbitals were attached to its lateral surface by 

 ligamentous tissue. Except for the connexion with the maxillary, this 

 description would apply in a very general way to the ectethmoid as described in 

 this paper. Stannius (1854) used the terminology of Cuvier in describing this 

 bone in the teleosts, and stated that it was one of the marginal bones of the 

 first and second head segements. 



Huxley (1864) remarked this bone as a development around the ectethmoid 

 process in Esox, and, though calling the ossification, the prefrontal, he stated 

 that it was comparable to the lateral mass ossification of the ethmoid bone of 

 human anatomy. Vrolik (1873) followed Huxley in using this name. Its 



