52 ILLINOIS BIOLOGICAL MONOGRAPHS [52 



and vein has been resorbed and the perichondria! osseous lamella forms the 

 floor of the canal in which they lie in the 32 mm. stage. The outer perichon- 

 drial ossification, together with the parietal ossification, forms the roof of the 

 canal, while the cartilage of the synotic tectimi forms its lateral walls. 



Near the anterior end of the otic capsules, the perichondrial lamellae on the 

 margins of the fontanelle interdigitate wdth the frontalia (Fig. 3). There is no 

 lateral line ossification an>^where near the vicinity of the developing supraocci- 

 pital. 



The spina occipitis element of the supraoccipital is developed from the ossi- 

 fication of the fibrous sheet between the dorsal anterior muscle segments and 

 the connective tissue on the dorsal surface of these (Fig. 29), as in Salmo. Near 

 the anterior end of the spina occipitis the dorsal dermal sheet is very broad and 

 extends laterally above the anterior ends of the body muscles (Fig. 3). The 

 connexion of this sheet with the outer perichondrial lamella is affected by 

 numerous osseous trabeculae. The interval between the dermal sheet and the 

 perichondrial ossification gradually becomes greater more posteriorly, and the 

 trabeculae become limited to the apex of the occipital arch (Fig. 29). 



The dorso-lateral sides of the occipital arch are embraced by a pair of 

 osseous plates, posterior to the otic capsules and separate from the cartilage 

 of the arch by fibrous connective tissue (Fig. 29). These plates enclose 

 the neural arch of the third vertebra which ensheathes the posterior end of the 

 occipital arch and are fused with the perichondrium of the former farther 

 posteriorly. These may be the representatives of an ossification comparable 

 to the proatlas of the reptiles. 



The maxillary region. The premaxillary ossification lies in the same place 

 as in the younger stage and differs only in its extent and in the greater number 

 of teeth attached to the ventral surface (Fig. 4). The maxillary ossification 

 is larger, and, as before, forms a case for the proximal end of the maxillary bar- 

 bel cartilage. It is attached to the lateral surface of the palatine cartilage, 

 (Fig. 4). 



Mandibular and suspensorial apparatus. The palatine cartilage has per- 

 sisted to a great extent, but there are regions anterior to its articulation with 

 the ectethmoid where the perichondrium has ossified and small osseous proc- 

 esses of bone project from its periphery (Fig. 22). The anterior end of the 

 cartilage is large and spherical, tapering posteriorly as in the younger stage 

 (Fig 4), and it is flattest at the place of contact with the ectethmoid process. 

 A small knob of cartilage projects posteriorly from its anterior dorsal surface, 

 to serve as a support for the beginnings of the lacrimal (Fig. 3). Posterior to 

 the articular surface the palatine is held in position by a pair of muscles which ex- 

 tend from the ethmoid cartilage and are fastened to its ventral and dorsal sur- 

 faces respectively (Fig. 39). Schleip ('03) says that the palatine of the salmon 

 and trout is a mixed bone and that it has both perichondrial and dermal ele- 



