SKULL AND VISCERAL ARCHES IN LEPIDOSIREN AND PROTOPTERUS. 57 



as far forward as the middle of the eye. In the present stage (36 + ) this process has 

 grown much longer, curving downwards and then forwards into the upper lip, where it 

 fades away into a thick strand of connective tissue supporting the labial fold. It has 

 in fact the relations described and figured by Bridge in the adult (figs. 9, 16). 



In the interval between stages 34 and 36 + , the nasal capsules have developed in the 

 following way. From the anterior end of the internasal septum grow out a pair of 

 cartilaginous processes, which grow back to form the ventral rims of the basket-work of 

 the olfactory capsules. These processes are the cornua trabecularum. They fuse 

 behind with an independently formed nodule of cartilage, the suhnasal cartilage of 

 Bridge. Prior to the outgrowth of the trabecular cornua from the front end of the 

 internasal septum, four cartilaginous outgrowths appear from the dorsal edge of this 

 septum on each side. These grow outwards and downwards to fuse with the ventral 

 rims which have been formed by the cornua, and thus the fenestrated olfactory capsules 

 are constituted. 



Occupying the fold of mucous membrane in the angle between the middle and 

 posterior palatine tooth plates, and not in any way connected with any other skeletal 

 structure, is the rudiment of Bridge's upper labial cartilage (shown in fig. 16). 



Bridge homologises Huxley's * anterior upper labial of Ceratodus with his 

 (Bridge's) subnasal cartilage in Lepidosiren. In Ceratodus this cartilage is separate 

 from the nasal capsule. Comparison with the adult Lepidosiren led Bridge to suppose 

 that this independence was secondary, and that the name upper labial was in- 

 appropriate, the cartilage really representing a disjointed part of the olfactory capsule. 

 K. Furbringer was of the same opinion. The independence of this cartilage in the 

 young Lepidosiren, however, shows that Huxley was in all probability right in his 

 nomenclature. 



As regards Huxley's posterior upper labials in Ceratodus, Sewertzoff's discovery 

 that these are in the young Ceratodus continuous with the ethmoidal region of the 

 trabecula, seems to establish Rose's suggestion (quoted by Bridge) that these represent 

 the ant-orbital processes of Protopterus and Lepidosiren, which have become separated 

 off in Ceratodus. (Sewertzoff himself does not discuss Rose's suggestion.) Hence 

 there is no cartilage in Ceratodus corresponding with the posterior upper labial in the 

 Dipneumona (the only labial cartilage allowed by Bridge). The condition of the labial 

 cartilages in the Dipnoi is then as follows : — All three genera have anterior upper 

 labials, which in Ceratodus remain free throughout life, but in the Dipneumona t fuse 

 with the olfactory capsules during the larval stage. Protopterus and Lepidosiren alone 

 have posterior upper labials. In Ceratodus these are wanting, but the ant-orbital 

 processes get separated from the trabeculse during development and simulate the 

 posterior upper labials of the other two genera. 



* Proc. Zool. Sot: Lond. ]876. 



t Owing to the larger gaps between the successive stages of Protopterus available, I have been unable to prove 

 the original independence of this cartilage in this genus. 



TRANS. ROY. SOC. EDIN., VOL. XLV. PART I. (NO. 3). 8 



