44 ILLINOIS BIOLOGICAL MONOGRAPHS [44 



the centre of the basioccipital on the ventral surface of the parachordal plate 

 is thick and spongy, and continuous with the ossification surrounding the 

 notochord. The transcapular processes of the shoulder girdle are fused with 

 the perichondrial ossifications on the lateral surface of the posterior end of 

 the parachordals, 



McMurrich (1884b), from a study of young Amiurus, 20 to 38 mm. long, 

 concluded that there never was a preformation of cartilage in the floor of the 

 sacculi in this region. Cartilage occurred here in the 38 mm. stage, but he 

 stated that it was due to the growth anteriorly of the cartilage at the very 

 posterior end of the cranium. I have observed cartilage in this region in the 

 10 mm. stage, and the ossification at the 32 mm. stage which forms the floor 

 of the recessus sacculorum is the result of the perichondrial ossification of the 

 original cartilage, the cartilage itself having been resorbed. 



The anterior processes of the scaphia, which, in the 10 mm. larva, lie 

 immediately dorso-lateral to the notochord (Fig. 12), are now separated from 

 it by paired cartilaginous masses (Fig. 37), the posterior continuation of the 

 posterior parachordalia. These are covered ventrally with the perichondrial 

 anlage of the basioccipital and latero-dorsally with that of the exoccipital. 

 Posterior to this region the notochord is relatively much larger than it is 

 intercranially. The endorhachis which supports the spinal cord and divides 

 the sinus impar into the atria sinus imparls is more compressed than in the 

 younger stage. It widens posteriorly in the region where the anterior proc- 

 esses of the scaphia fuse with the scaphia proper (Fig. 29), which are as yet 

 of cartilage and articulate by rounded surfaces with the notochord. The 

 claustra are better defined than in the younger stage; they lie between the 

 scaphia and the anterior end of the third neural arch, forming the wall of the 

 vertebral canal in this region. The second post-vagal nerve issues posterior 

 to the scaphium (Fig. 35). The most striking morphological feature of this 

 region is the posterior growth of the parachordalia and the subsequent separa- 

 tion of the anterior processes of the scaphia from the notochord (Fig. 37). 

 The enclosure of the hypoglossus nerve within the cranium by ossification 

 (Fig. 23) is comparable with the history of the same nerve in Gasterosteus 

 (Swinnerton, 1902) and the first two post-vagal nerves of Salmo (Harrison 

 1895; Willcox, 1899). 



Schleip (1903) has described the formation of the ossifications around the 

 parachordal cartilages and the ventral parts of the otic capsules in Salmo. He 

 describes the basioccipital ossification as arising from paired inner and outer 

 lamellae on the parachordals in the region of the fenestra basicranii posterius, 

 into which the notochord projects. Anterior to the notochord the inner and 

 outer lamellae meet across this fenestra, but in the region of the notochordal tip 

 they are separated from each other by an osseous mass around the notochordal 

 sheath, which he calls the ' ausfiillende Knochenmasse. ' In Amiurus there is 

 no fenestra around the anterior tip of the notochord and the ossification on 



