116 TRANSACTIONS OF THE AMERICAN PHILOSOPHICAL SOCIETY 



of the braincase are connected with the following contrasts in the brain and sense organs, 

 among others pointed out by Watson (1925): 



Shark Primitive actinopteran 



Olfactory parts Very large Reduced 



Optic parts Moderate Much enlarged 



External rectus muscle of eye Short Produced backward (forming myodome) 



Cerebellum Moderate to large Much enlarged and involuted 



The olfactory or ethmoid region of the neurocranlum of the palaeoniscids figured by 

 Watson is much less expanded anteroposteriorly than it is in typical teleosts. Processes 

 homologous with the lateral ethmoids (parethmoids) of teleosts were present but not sep- 

 arately ossified from the general mass. 



The occipital segment of the palaeoniscoid cranium (Watson, 1928, Figs. 1-3; 1925, 

 Fig. 12), consisting of the basioccipital and an ascending occipital plate, represents the neo- 

 cranium or segmental portion of the cranium (Fiirbringer, 1897; Gaupp, 1906) which is 

 here shown in a very primitive stage. The possession of a neocranium is characteristic of 

 the ganoids and teleosts, in contrast with the elasmobranchs, which have only a palseo- 

 cranium, not extending behind the vagus nerve (Fiirbringer, as quoted by Kindred, 1919, 

 p. 23). 



According to the views of Traquair, Smith Woodward and others, the Palaeozoic 

 palaeoniscoids gave rise to the following groups: (1) the very short and deep-bodied Platy- 

 somidse, which have a downwardly directed suspensorium and very small nibbling mouth; 

 (2) the Trissolepidse of the Permian; (3) the very peculiar Carboniferous genus Phanero- 

 rhynchus of E. L. Gill (1923^), which is a palseoniscoid with a cartilaginous rostrum like 

 that of Acipenser and a number of progressive semionotid-like characters in the fins (Watson, 

 1925); (4) the long-jawed Saurichthyidae of the Triassic (cf. Stensio, 1925); (5) the Jurassic 

 Chondrosteidae, which appear to be close to the lines leading to (6) the sturgeons and (7) 

 the spoonbills; besides all these there were (8) the progressive Catopteridas of the Triassic, 

 which are normal fusiform fish with a fair-sized mouth. Neither the Catopteridae nor any 

 other known family of Chondrostei, however, appear to be directly ancestral to the typical 

 holostean or protospondylous ganoids and later teleosts. 



In the platysomids (as described chieiiy by Traquair, 1879) the form of the head was 

 clearly subordinate to the great depth of the body as a whole (Fig. 15) and in the extreme 

 forms some of the skull plates are greatly lengthened vertically. This line of specialization 

 finally culminates in extremely deep-bodied forms and the series as a whole has every ap- 

 pearance of being derived from such primitive fusiform types as Cheirolepis and Palceoniscus. 

 In the Catopteridae of the Triassic the skull pattern approaches that of the contemporary 

 ancestors of the higher ganoids but only by way of convergence. Here again we have evi- 

 dence that a short or moderate mouth and downwardly directed suspensorium have been 

 derived from a large mouth and backwardly inclined suspensorium. 



After a most painstaking description and analysis of the skull characters of the saurich- 

 thyids (Fig. 16), Stensio concludes that they must be closely related to the palaeoniscids, 

 as maintained by Woodward since 1895 (p. vii); that while in some respects they are more 

 primitive than the palaeoniscids (e.g., in the retention of a quadratojugal, lost in the latter); 

 on the whole they are considerably more specialized than these. Stensio concludes (1925, 

 p. 223) that "among the Chondrostei the saurichthyids are closely related to both palaeonis- 



