COAL-MEASURE STEGOCEPHALIAN. 957 



of "Loxomma" in its structure, particularly in the fact that the 

 splenial is entirely a bone of the outer side of the jaw, which 

 is pi'oved in the most conclusive manner by specimen III. 



In later Stegocephalia the splenial (as shown, for example, 

 in Eryops, Anaschisma, Botliricej)s^ and " Labyrinthodon " lepto- 

 gnathus), although it still has a good exposure on the outer 

 surface, has also a large flange running up on the inner side of 

 the jaw to the coronoid 



In Reptilia the internal part of the splenial is always larger 

 than the external, and in later reptiles this latter part disaj)pears 

 altogether. 



It thus seems certain that the trend of evolution of the splenial 

 has been directed to gradual growth of an internal flange at the 

 expense of the external part of the bone which alone occurs in 

 the primitive amphibia. 



In correlation with this development is a reduction of the 

 coronoid and a complete loss of the epicoronoid. 



As the angular is to all appearance a serial homologue of the 

 splenial, it is probable that it may have tended in early Amphibia 

 to pursue a similar course of change, in which case we may 

 suppose that in primitive forms the suture between the angular 

 and the prearticular probably ran along the lower border of the 

 jaw just as does the sutui'e (when there is one) between the 

 coronoid and the splenial. 



This type of jaw, which we have arrived at hypothetically, is 

 actually realized in Crossopterygian iishes, MegaUchthys being the 

 best type. 



The structure of the jaw in Palaeozoic Crossopterygian s was first 

 accurately made out by Traquair in Bhizodus and lihizodopsis. 



I give here an account of the jaw oi Megaliclithys and have also 

 examined those of HolojJtychhis and Osteolepis, which agree in 

 general structure. 



In Megalichthys the dentary is a large bone narrowing from 

 front to back, where it ends in a point. It bears throughout its 

 border a single row of small pointed teeth, within which at the 

 anterior end are one or two large ^'laniary" teeth. Its lower 

 border overlaps three bones, the splenial, angular, and sur- 

 angular. 



The splenial has an articulation with its fellow ; it lies entirely 

 on the outer side of the jaw and its lower border is entirely free. 

 Its oblique posterior edge overlaps the angular. 



The angular is a bone similar to the splenial, except that it 

 has of course no symphysis and its lower border articulates with 

 the coronoid. 



The surangular is overlapped by the angular, which it greatly 

 resembles ; it covers the outer side of the well-ossified articular. 



* The combined prearticular and coronoid is a very large bone 

 running forward from the inner side of the articular nearly to 



* A specimen of Grlyptolepis paucidens in the Manelioster Museum seems to 

 show a clear suture between the short prearticular and the coronoid. 



Proc. Zool. 8oc.— 1913, No. LXIY. 64 



