63] THE SKULL OF AMIURUS— KINDRED 63 



The frontal bone extends down into the wall of the orbit, externally con- 

 necting by suture with the orbito- and alisphenoid ossifications (Fig. 20). In 

 the anterior part of the orbit it is separated from the ectethmoid bone by a small 

 remnant of the fused alisphenoid-ectethmoid cartilage. Posteriorly it inter- 

 digitates with the anterior end of the sphenotic bone, above the aUsphenoid. 

 On the inner surface of the cavum cranii, the frontal descends in the cranial 

 wall in front of and behind the line of suture between the two frontals, but 

 immediately below the suture this downgrowth is limited by the dorsal parts of 

 theali- and orbitosphenoids. Anteriorly it overlaps the orbitosphenoid and con- 

 tinues as far forward as the cartilage which lines the cavum within the ecteth- 

 moid. Posteriorly it overlaps the alisphenoid, interdigitates with the anterior 

 margin of the sphenotic and proceeds dorsally toward the middle hne of the 

 cranium to interdigitate with the supraoccipital. 



There are many minute nerve foramina and canals in the frontal bone, but 

 none of them reach the cavum cranii. The largest canal is that of the ophthal- 

 micus superficialis faciahs, which, after issuing from the alisphenoid, sends a 

 branch along the dorsal median wall of the orbit and is enclosed within the 

 frontal. It continues forward within the bone, sending small twigs dorsally 

 into the bone to the lateral line organs contained within it and also clear through 

 the bone to the in tegumental sense organs above (Fig. 10). Anteriorly, the 

 nerve passes to the dorsal surface of the bone through a small foramen, just 

 below the insertion of the connective tissue which connects frontal with nasal. 



The supraorbital lateral line canal, which starts in the nasal bone, enters 

 the anterior end of the frontal after passing through the connective tissue 

 between the two bones. The anterior point of ingress of the canal lies just dorsal 

 to the foramen for the passage of the ophthalmic branch of the facialis. From 

 this point its course cannot be followed externally, but must be traced by fol- 

 lowing it through a series of transverse sections, (a 60 mm. specimen was used 

 for this). There is one dermal tubule and pore anterior to the frontal ridge 

 on the dorsal surface of the bone. As noted above, the suborbital canal enters 

 the frontal at the anterior end of the frontal ridge. It anastomoses with the 

 supraorbital canal beneath the ridge and a dermal tubule extends along the 

 posterior face of the ridge from the supraorbital canal, just before their anas- 

 tomosis. The canal formed by the anastomosis continues posteriorly from the 

 middle of the frontal ridge and thence into the sphenotic bone (Fig. 11). There 

 are three sense organs in the canal enclosed in the frontal at points shown in 

 Figure 11. The first and third are followed by dermal tubules, but the second, 

 which lies just anterior to the union of supraorbital and suborbital canals is not. 



This bone of the adult is the result of the ossification of the membrane above 

 the alisphenoid cartilage of the larva. In sections through the 60 mm. larva 

 the fibrous connective tissue surrounding the ventral surface of this membrane 

 have ossified and appear as lamellae capping the perichondrial ossification of 

 the dorsal surface of the alisphenoid cartilage (Fig. 32). The ossifications of 



