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



appear that the ventral or symplectic extremity of the hyomandibular element may be 

 segmented off as a separate cartilage, and further, that an additional nodule of cartilage, 

 which is occasionally found in the hyo-suspensorial ligament, " may possibly have the 

 value of an interhyal" (I. c. p. 636). No trace of any patches of cartilage or 

 "vestigial hyoidean rays" could be detected in connection with the cerato-hyal in 

 the only specimen of Ceratodus which I have had the opportunity of examining. 



Ceratodus has five branchial arches, with a rudiment of a sixth if Huxley has 

 correctly interpreted the nature of a small nodule of cartilage at the ventral extremity 

 of the fifth arch. The first four are bisegmental, consisting of a " long ventral and a 

 short dorsal piece of cartilage " ; the last, of a single cartilage only. Two small cartilages 

 situated in the median and ventral line, between the ventral extremities of the branchial 

 arches, have been considered by Huxley to represent " anterior and posterior meso- 

 branchials " (I. c. p. 27). 



There is a well-developed hyoidean cleft between the hyoid arch and the first 

 branchial arch. 



A representative of the " cranial rib " of other Dipnoi is present, but its relations to 

 the ossified " basi-dorsals " of the first neural arch are precisely those of the succeeding 

 ribs to their basi-dorsals. 



The attachment of the dorsal extremity of each lateral half of the pectoral girdle to 

 the skull is effected by " a thin broad lamella of an obliquely ovate shape, entirely 

 ossified" (Giinther, I.e. p. 531), and directed obliquely backward and outward. It 

 is directly connected by ligament at its upper or inner extremity with the lateral wall 

 of the chondrocranium, between the Vagus foramen and the exoccipital, and distally 

 also by ligament with the supraclavicle of Parker. The bone has been termed " supra- 

 scapula " by Giinther, and its equivalent in Protopterus " suprascapula " (Peters, I. c.) 

 and "post-temporal" (Parker, 32). 



B. Protopterus. 



With the exception of minor structural features, and certain modifications of 

 nomenclature, the data necessary for the comparison of the skull of this Dipnoid with 

 those of Ceratodus and Lepidosiren have been principally taken from Wiedersheim's 

 paper (I. c). 



Owing to the development of a large fronto-parietal bone and the share which it 

 takes in forming the roof, and also, in conjunction with the parasphenoid, the lateral 

 walls of the cranial cavity, the chondrocranium has largely atrophied, leaving, neverthe- 

 less, an occipito-periotic portion and an ethmo-nasal region, connected by a pair of 

 laterally-situated and relatively narrow, band-like, trabecular cartilages, which in the 

 middle portion of the skull are the only remains of the complete chondrocranium of 

 Ceratodus. The occipito-periotic cartilage is somewhat more extensive than in Lepido- 

 siren, the cartilage of the supraoccipital region extending on the inner surface of the 



