308 tUOF. T. W. BRIDGE ON IHB [-A-pr. 2, 



externally, and the frontal (/.) and ethmo-nasal {ei.n.) bones 

 above. The bones forming the lateral portions of the rostrum or 

 snout — that is, the pterygoid, the superficial palatine, and the 

 divided maxilla — are firmly connected together, but their connection 

 with the mesial elements of the rosti'um — the parasphenoid, 

 vomers, and ethmo-nasals — is loose and ligamentous. The usual 

 ethmo-palatine articulation is apparently altogether wanting in 

 Lejjidosteus ; but the necessary lateral mobihty of the palato- 

 pterygoid series in fe-ont is secured, (i.) by the flexibihty of the 

 thin anterior portion of the pterygoid, and (ii.) by the loose liga- 

 mentous connection which has been described as existing between 

 the palato-pterygoid series, including the maxillae, and the mesial 

 bones of the beak. The place of the normal ethmo-palatine joint 

 is apparently taken by a secondary articulation furnished by a 

 smooth oval surface on the inner side of a preorbital process of 

 the frontal and a similar facet on the superior border of the 

 pterygoid. It may be concluded, therefore, that, as in Osteoglossum, 

 the palato-pterygoid series of bones are capable of more or less 

 extensive lateral movement as a i-igid whole on the metapterygo- 

 parasphenoidal and the pterygo-frontal articulations, and, further, 

 that such lateral movements are accompanied by the alternate 

 approximation and separation of the two pterygoid bones in the 

 mesial line of the oral cavity. 



As regards the character of the teeth supported by the oral 

 bones, Lejyidosteus dififers considerably from Osteoglossum. Apart 

 from the acutely pointed teeth in the maxillae, there is a series of 

 similar but much smaller teeth in the superficial palatines, and also 

 a number of closely-set denticles on the vomers and on a limited 

 area of the oral surface of each pterygoid. These teeth, however, 

 can only be of service in the vertical movements of the ordinary 

 jaws, and even in the case of the pterj'goid teeth it is obvious, 

 from their position, size, and mode of implantation, that they can 

 have no functional significance in connection with the lateral 

 mobility of their supporting bones. It is possible that, as in Esox^ 

 the lateral mobility of the palato-pterygoid bones and the maxillae 

 is simply to admit of the lateral expansion of the jaws when 

 seizing relatively large prey ; but although this may be one of the 

 advantages which the fish derives from this mechanism, the 

 probability that the partially swallowed prey may be subjected to 

 a process of crushing in the oral cavity by the lateral movements 

 of the pterygoids must, nevertheless, not be lost sight of. It may 

 also be remarked that the latter suggestion is strongly supported 

 by the fact that the superior or inner edges of the two pterygoid 

 bones are capable of being brought into actual contact in the 

 median Hne of the mouth-cavity for a considerable portion of 

 their length. 



There is another interesting feature in which Lepidosteiis 

 resembles Osteoglossum, and that is the structure of the " tongue." 

 In the Ganoid the floor of the mouth is elevated to form a well- 

 marked, elongated, and somewhat strap-shaped " tongue," which 



