362 PHOF. T. W. BRIDGE ON THE MOEPHOLOaY OF THE SKULL 



describes is identical with the one which, in agreement with Wiedersheim, I have 

 described as existing in front of the first cleft, although it is extremely probable that 

 such is the case ; but I entertain no doubt whatever that the latter cartilage is 

 the first branchial arch and that the hyobranchial cleft has been completely suppressed, 

 and further that, as in Lepidosiren, the hyoidean hemibranch or pseudobranch is in 

 relation with the anterior wall of the cleft between the first and second branchial 

 arches. 



In Wiedersheim's description all the remaining five branchial arches are represented 

 as simple, non-jointed, and very slender rods of cartilage. The second arch is the 

 longest, but is scarcely thicker than the othei's, neither is its dorsal end bifurcate or 

 grooved. The remaining arches gradually decrease in length, the sixth being much 

 the shortest of the series. According to Ridewood (/. c), on the contrary, the second 

 and third branchial arches diff'er from all the others, and resemble the branchial arches 

 of Ceratodus, in tliat each is provided with a dorsal or " epibranchial " segment in 

 addition to the relatively much longer ventral or cerato-branchial portion which, 

 apparently, is alone represented in Wiedersheim's figures. Meso- or basi-branchial 

 cartilages appear to be wholly absent in Protopterus. 



The lower jaw is strikingly different from that of Ceratodus. The two rami are 

 relatively shorter and more strongly developed, with a remarkably high coronoid 

 process, and uniting anteriorly in a massive symphysis. As in Lepidosiren, practically 

 the whole of the osseous portion of each ramus is formed by the splenial, the dentary 

 being absent altogether, and the angular reduced to an insignificant splint on the outer 

 surface of the articular extremity. The Meckelian cartilages unite anteriorly in a 

 vertically-disposed symphysial plate, terminating above in a median and two lateral 

 processes. According to Wiedersheim [I. c), a short ascending process is given off 

 from each cartilage and occupies the angle between the anterior and central tooth- 

 plates. I notice, however, that in one of Peters's figures of his Zambesi Protopterus 

 the Meckelian cartilage is represented as giving off an additional process behind the 

 one referred to above, and, moreover, both are represented as if they were suturally 

 distinct from the cartilage itself [1. c. Taf. ii. fig. 5). The additional process is 

 apparently represented by a free nodule of cartilage in Lepidosiren (PI. XXVIII. 

 fig. 7). 



In Profopterus the " cranial rib " has become displaced from its normal relations to 

 the elements of the first neural arch, and moved forward to an articulation with the 

 chondrocranium between the Vagus foramen and the exoccipital. 



The foramina for the major divisions of the cranial nerves have the same general 

 arrangement as in Lepidosiren. It need only be mentioned that each Optic nerve 

 escapes through a small foramen in the fibrous sheet wliich closes the lateral cranial 

 vacuity of its side, and that the foramen for the Motor Oculi perforates the side-wall 

 of the skull at some distance behind the Optic nerve, but anteriorly to the aperture for 



