454 



CONTRIBUTION TO THE 



in the dermal bones. The rudimental mosopterygium shown by Gegenbaur to exist in the young nlwridm, the prcoco- 

 racoid arch and the ventral fins, are shared with, the sturgeons and other divisions. 



There are considerable varieties of type in subordinate features to be observed in the Ncmatognathi. The Hypo- 

 phtJialmidm are indeed scarcely to be referred to this order. 



1 Anterior vertebrae modified ; inferior pharyngeal bones distinct ; few branchiostogal radii ; 

 Operculum present. Siluridm. 

 Oporculum wanting. Asprediriidm. 



2 Vertebrae unmodified ; inferior pharyngeal bones united their wholo length many branchiostegal rays ; 

 Operculum present, ' ' HypopMhalmidm. 



In the Siluridm we have other modifications of importance. Tims in Flotome the second superior pharyngeal 

 is wanting. In Loricaria and its allies, the pterotic is greatly expanded, so as to unite witli tho hyomandibular and 

 opercular bones. 



ScYrnornoRl. Parietals narrow, distinct from each other and the supraoccipital. Pterotic large funnel-shaped, 

 enclosing a chamber which expands externally, and is covered by a, lid-like bone. No symplectic. Opercular bones 

 present. Anterior vertebrae unaltered. No mesoptorygium. Basis cranii simple. No interclaviclcs. 



This very distinct group is about equally allied to the preceding and the following. The arrangement of tho parie- 

 tal bones appears to explain thai, of the Siluroids, about which there is some difference of opinion among naturalists. 

 Thus Huxley in his memoir on the Ganoids, in Ids figure of Olarias, labels these confluent elements (as I suppose 

 them to be at present), supraoocipitals ; Parker (shoulder girdle), calls them parietals. The families of this order 

 are two, viz: 



Tail isocercal, GymnarcJiidm. 



Tail diphycercal, Mormyridce. 



In Morrnyrw there is one vertebra of the original series left in tho tail. 



Plectospondyli, Parietals broad, distinct,, pterotic normal, symplectic present. Opercular bones all present; 

 no interclavicles. Anterior four vertebra! much modified and with ossicula audilus. 



1 Brain case produced between orbits ; only two superior pharyngeal bones; basis cranii simple. 

 a Only two basal branchihyals. 



Tail diphycercal, Catostomida. 



a a Three basal branchihyals, 



f Gyprinidm. 

 Tad diphycercal, ( (Jobitidm. 



Tail isocercal, Stemoptfgidm. 



2 Brain ease not produced between orbits, basis cranii double, with muscular canal in many ; four to one superior 

 pharyngeal bones. 



r l hree basal branch ihyals, ^ ErytJvrinidce. 



Tsosi'ondyu. Parietals separate, symplectic present; no interclaviclcs. Anterior vertebras simple, unmodified. 

 Pharyngeal bones separate above and below. 



An extensive group, presenting many subordinate modifications. 



1 Tail subisoccrcal ; pterotic externally annular enclosing a largo cavity ; basis cranii double ; Nolopteridct. 



2 Tail diphycercal; pterotic normal, basis cranii double; superior pharyngeals four, distinct, third largest directed 

 forward ; basal branchihyals three : (typical.) 



* Parietals united. 



