29] THE SKULL OF AM lURUS— KINDRED 29 



In all parts of the cranium the cartilage is more massive and there are very- 

 few places where ossification has proceeded far enough to replace it entirely. 

 The fenestra hypophyseos (Fig. 3) is narrower and is closed ventrally by the 

 elongate parasphenoid ossification (Fig. 4). The cranium has grown 6 mm. in 

 length since the 10 mm. stage, and is relatively much flatter. Detailed de- 

 scriptions of the various cranial regions follow. 



The ethmoid region. This part of the cranium differs considerably from 

 the younger stage. The olfactory lobes are no longer in the very anterior 

 region of the ethmoid plate, lateral and internal to the olfactory foramina 

 (Fig. 30), but lie farther posteriorly, and a massive internasal septum has 

 grown up between them (Fig. 22). The foramina, instead of lying in an antero- 

 posterior plane parallel to the long axis of the body as in the 10 mm. stage, now 

 lie almost at right angles to it (Fig. 3). 



The ethmoid cornua, formerly wide blunt processes separated by a slight 

 indentation, are now narrow and pointed, with a deep notch between them 

 (Fig. 3). The floor of the nasal fossa (Fig. 22), the solum nasi of Gaupp, is 

 wider and thicker than in the 10 mm. stage, although even now it does not 

 extend as far laterally as the palatine cartilage. The ectethmoid process 

 (Figs. 3, 4) described earlier as projecting from the cranial wall at the junction 

 of the ethmoid and alisphenoid cartilages, forms an oblique ridge in the cranial 

 wall above the orbital foramen (Fig. 39) and the anterior part of the orbit. 

 The foramen orbitonasale is more posterior, evidence that the cranial parts 

 anterior to it have elongated. The process in the dorsal part of the cranial 

 wall has groMTi medially and has fused with the internasal septum to form a 

 rudimentar}^ tegmen cranii (Fig. 3). 



The anterior margin of the fenestra hypophyseos, formed by the fusion of 

 the anterior ends of the trabeculae, lies midventral to the optic foramina (Fig. 

 4), and farther posterior than in the younger stage. The orbital foramen is in 

 approximately the same position as in the younger stage, but the posterior 

 dorsal margin of the ectethmoid process now lies above it (Fig. 39). The 

 foramen is limited by a perichondrial ossification between its ventral and dorsal 

 margins, so that a small aperture is all that remains of the larger foramen of 

 the 10 mm. stage. These ossifications are continuous anteriorly with the 

 perichondrial ossifications on the posterior wall of the ectethmoid process. 



A perichondrial ossification on the dorsal margin of the olfactory foramen 

 encloses a branch of the internal carotid artery passing from the cranium to 

 the nasal sac. The olfactory tract, from lobus to the olfactory foramen, is 

 entirely enclosed within the cranium. The development from the condition 

 in the younger stage, has kept the lobus close to the brain, while the anterior 

 end of the cranium grew forward and pulled the olfactory organ with it, result- 

 ing in an elongate tractus olfactorius. This stage of tlie development of the 

 olfactory relations is comparable to that of the Selachian type as has been 

 pointed out in the Cyprinidae by Sagemehl (1891) 



