360 PROP T. W. BRIDGE ON THE MORPHOLOST OF THE SKULL 



A noteworthy feature in Proiopterus is the existence of extensive lateral vacuities in 

 the side-walls of the anterior section of the cranial cavity, between the anterior margins 

 of the lateral plates of the fronto-parietal bone and the ascending processes of the 

 palato-pterygoids, which are continuous dorsally with a fissure between the fronto- 

 parietal and the dermal ethmoid. These vacuities are filled up by a tough fibrous 

 membrane, continuous internally with a dense mass of fibrous tissue previously 

 mentioned as obliterating the cranial cavity in this region, while the dorsal fissure is 

 closed by the fibrous tissue which extends between the contiguous margins of the 

 fronto-parietal and the dermal ethmoid, and is also continuous internally with the 

 cranial mass of the same tissue. 



The parasphenoid is much more restricted in its relations to the basis cranii than in 

 Ceratodus, inasmuch as it terminates somewhat abruptly by a nearly straight transverse 

 margin at some distance behind the palatine symphysis, and only very slightly projects 

 backward beneath the post-cranial section of the notochord. In the share which it 

 takes in forming the lateral walls of the cranial cavity, and in its relation to the 

 trabecular cartilages and the palato-pterygoid bones, the parasphenoid differs but little 

 from its homologue in Lepidosiren. 



The palato-pterygoids are also essentially similar to those of Lepidosiren, especially 

 in the modifications produced in the ethmo-nasal region of the chondrocranium by the 

 massive development of their symphysis. They have well-marked lateral processes for 

 the partial support of the two anterior tooth-plates, and a strong " antorbital process " 

 for the posterior plate. There are also stout ascending processes which, in addition to 

 carrying the dermal ectethmoids, form the outer walls of the olfactory fossae anteriorly 

 to the lateral cranial vacuities. 



There is a ring of fibrous tissue encircling the orbit, but no circumorbital bones are 

 developed in it. The opercular and interopercular elements are essentially similar to 

 those of Ceratodus, but relatively smaller. 



Vestigial hyoidean rays have not previously been recorded as existing in ProtojJterim, 

 but there is no doubt as to their existence. The opercular cartilage closely 

 resembles the corresponding piece in Ceratodus (Huxley, I. c. p. 38). It is horseshoe- 

 shaped and closely applied to the inner surface of the upper third of the operculum. 

 The interopercular cartilage, on the other hand, is essentially similar to its representative 

 in Lepidosiren. 



The hyoid arch is a much simpler structure than in Ceratodus, and, as in Lepidosiren, 

 there is no trace of hyomandibular, hypohyal, or basihyal elements. As in the latter 

 Dipnoid, there is a patch of cartilage (vestigial hyoidean ray) on the outer surface of 

 the distal portion of each cerato-hyal. No mention of this cartilage is made by 

 Wiedersheim, although it is apparently figured by Peters [l. c. Taf ii. fig. 2), without, 

 however, being indicated by reference-letters or referred to in the text. 



As regards the ])recise number of branchial arches in Protopterus, there is some 



