134 TRANSACTIONS OF THE AMERICAN PHILOSOPHICAL SOCIETY 



type. All the derm bones have lost completely the ganoine covering that was so char- 

 acteristic of their Mesozoic ancestors, while, as above noted, the postorbital or second series 

 of plates has practically disappeared. The ethmoid region is extended anteroposteriorly 

 and includes distinct lateral ethmoid ossifications. A special point of resemblance to the 

 clupeoid isospondyls is the presence of posttemporal fenestrae in the occipital surface 

 for the insertion of prolongations from the trunk musculature. The neurocranium, as 

 described by Allis (1895, 1897a), also approaches the primitive teleost type. There is a 

 distinct though small myodome and the parasphenoid is prolonged backward beneath the 

 occiput. 



At the same time Jmia retains a number of ancient ganoid characters, especially: 

 the small size of the mesethmoid, the large size of the paired bones in front of the frontals 

 (probably rightly called nasals), the paired condition of the "vomers" (= prevomers), 

 and the presence of an antorbital plate and of so-called splenial (= coronoid and pre- 

 articular) plates in the mandible. 



\'ery probably the presence of so much unossified cartilage in the chondrocranium is 

 not a primitive character but a retrogressive feature and a retention of larval characters 

 in the adult. 



In short, Jmia, in respect to its skull structure as well as in most other parts of its 

 anatomy, stands much nearer to the base of the teleosts than it does to Lepidosteus, with 

 which it was formerly grouped as a "ganoid." 



The skull of Caturus as figured by Smith Woodward (1897, Pis. VIII, IX) appears to 

 afford an ideal structural ancestor to the Jmia type, and approaches the latter in many 

 characters of the skull-top face and mandible. 



