10 Dr. R. H. Traquair on the Genus Dipterus. 



of the skull and overlapped by the operculum. This is suc- 

 ceeded bj a clavicle, whose direction is first downwards and 

 then somewhat forwards. The lower part, forming an obtuse 

 angle with the upper, is formed, as in Ceratodus^ of a distinct 

 piece, divided off from the upper by a suture which passes just 

 below and in front of the attachment of the pectoral fin, and 

 may be regarded as the equivalent of the infraclamcular 

 bone in Crossopterygii and Acipenseroidei. 



The points of resemblance between Dipterus and Ceratodus 

 certainly do not stop when we come to the paired fins, which, 

 so far as their structive is preserved in the former genus, are 

 similarly conformed in both. There is, in Dipterus, a central 

 elongated and pointed scaly axis, fringed, both preaxially 

 and postaxially, with a series of delicate fin-rays. In no 

 case is the internal skeleton preserved; hence we may truly 

 infer that, as in Ceratodus, it was cartilaginous ; and I can- 

 not conceive of any reasonable doubt as to its having been 

 also archipterygial in its nature. Mr. L. C Miall, who 

 seems to entertain doubts on this point, says that " the pre- 

 sence or absence of scales upon the fin does not mean 

 much," though in another passage already quoted he thinks 

 it " probable " that Dipterus and Ctenodus have an archi- 

 pterygium. The question is not, however, one of the pre- 

 sence or absence of scales upon a fin, but of the arrangement 

 of the rays. In many specimens of Dipterus the covering 

 of scales upon the axial " lobe " of the pectoral and ven- 

 tral members is so delicate and thin that the rays stand 

 out boldly defined for their entire length, the pre- and 

 postaxial series enclosing between them an elongated pointed 

 space, which it is as reasonable to conclude was once occupied 

 by a cartilaginous skeleton as that the orbit of a fossil fish 

 once contained an eye with lens and retina. And as in the 

 only recent form ( Ceratodus) in which the rays have that re- 

 markable arrangement this skeleton is archipterygial, we are 

 scarcely chargeable with rashness in believing that this also 

 was the case in Dipterus, 



The vertebral axis was in Dipterus certainly notochordal, 

 as shown by numerous specimens in the Edinburgh Museum ; 

 and from what is seen of its ribs, vertebral apophyses, and 

 interspinous bones or fin-supports, they were conformed and 

 arranged much as in Ceratodus. Pander, indeed, described 

 and figured certain detached vertebral bodies which he doubt- 

 fully assigned to Dipterus', but all the evidence is to the contrary. 

 In many specimens, however, as has been already noted by 

 Pander, there is to be seen in the body-axis, in the region of 

 the caudal fin, a raised line of a somewhat beaded appearance. 



