350 PKOF. T. W. BRIDGE ON THE MORPHOLOGY OF THE SKULL 



fine branch, which, after a short course obliquely downward and forward across the 

 outer surface of the " cranial rib," joins a slender branch of the Vagus, The latter 

 nerve evidently corresponds to a similar nerve in Protopterus, described by Pinkus [1. c. 

 p. 325) as being peripherally distributed to the tongue-rnuscles and to the musculature 

 of the pharyngeal wall. Hence it follows that, although a ramus communicans does 

 connect the Hypoglossal with the Vagus-group of nerves, yet the brachial nerve 

 contains no Vagus fibres ; and further, we have the interesting possibility that the 

 fibres of the Hypoglossal contribute both to the formation of the brachial plexus and 

 to the innervation of the muscles of the tongue. 



TJie Larger Mood-vessels and the Cranial Foramina. — An examination of the relations 

 of some of the larger blood-vessels to the cranial foramina yielded results substantially 

 agreeing with the previously recorded statements of Hyrtl [l. c). 



The superior jugular or anterior cardinal vein emerges from the cranial cavity 

 through the Vagus foramen (PI. XXVIII. fig. 1, x.), and is at once joined by the 

 inferior or external jugular vein. 



The internal carotid artery perforates the base of the skull near the outer margin of 

 the parasphenoid (PI. XXVIII. fig. 3, in.c), and ultimately enters the cranial cavity 

 through a small foramen near the inferior margin of the Gasserian recess (PI. XXVIII. 

 fig. 6, in.c). After its origin from the first aortic arch the external carotid traverses 

 the " Schlafengrube," and, in company with the mandibular and maxillary branches of 

 the Trigeminus nerve, appears on the lateral surface of the skull. It then breaks up 

 into several branches for the supply of the temporal and masseter muscles and the 

 orbital region, and also sends a considerable branch to the lower jaw, which in its 

 course accompanies the ramus mandibularis of the Fifth nerve. 



III. Bevision and Comparison of the Structure of the Skull in Ceratodus, 

 Protopterus, and Lepidosiren. 



A. Ceratodus. 



In comparing the cranial characters of the three genera it will be advantageous and 

 certainly convenient to begin with the skull of Ceratodus. For the necessary data I 

 shall rely principally on the accounts of Giinther and Huxley, but in certain minor 

 details on the results of my own observations. 



The most obviously distinctive feature in the skull of Ceratodus is the retention 

 throughout life of a complete chondrocranium, forming the continuous dorsal, lateral, 

 and ventral walls of the cranial cavity. To this may be added the fact that the 

 ethmoidal and ^asal regions are better developed and more normal than in any other 

 Dipnoi. By way of illustration, as well as for the sake of comparison with other types, 

 a brief description of the ethmo-nasal region may be given. 



The anterior section of the cranial cavity is continuous with two tubular olfactory 



