yin 
INTRODUCTION. 
whole of the branchiostegal apparatus is wanting, while the opercular 
series is reduced to a single small operculum. If these are not 
degenerate Chondrosteans they must he abnormally modified 
Crossopterygians, as suggested by 0. M. Reis x . 
Degeneracy of the Palseoniscid type in the direction of the 
modern sturgeons is distinctly observed in the Chondrosteidee of 
the English Lower Lias. CJiondrosteus resembles the Palseoniscidse 
in the arrangement of its cranial roof and the opercular apparatus, 
except that the gular plate and possibly a few of the branchiostegal 
rays have disappeared; its trunk is like that of the abnormal 
Palseoniscid Phanerosteon, and equally similar to that of the modern 
Pohjodon ; its reduced toothless jaws are much simpler than those 
of the Palseoniscidae, and the premaxillae have disappeared as in all 
the modern sturgeons and paddle-fishes. A more clearly inter¬ 
mediate type can scarcely be conceived ; and the fact that the 
Polyodontidee at least once possessed a regular squamation is 
demonstrated by the Eocene genus CrossopJiolis. 
Peotospondtii. 
It has already been remarked that the Catopteridse of the Trias 
incline towards a higher type of fish than the Chondrostei, to which 
they technically belong; but the two known genera cannot be the 
ancestors of this more advanced race, for at least one of its repre¬ 
sentatives (Acentrophorus) has already been found in the Upper 
Permian, while numerous and varied forms are commonly met 
with in the Trias and Rhaetic. It can only be affirmed that as 
soon as six important modifications had simultaneously affected 
the Chondrostean skeleton, another vigorous race arose and a new 
impetus seems to have been given to variation. These changes 
comprised (i.) the almost complete atrophy of the upper caudal lobe, 
(ii.) the reduction of the dorsal and anal fin-rays to exactly the 
same number as their supports, (iii.) the disappearance of the 
infraclavicular plates, (iv.) the loss of the pelvic baseosts, (v.) the 
subdivision of the hinder expansion of the maxilla, and (vi.) the 
withdrawal of the preoperculum from its extension over the cheek. 
Numerous types, in some respects parallel to those already noticed 
among the Chondrostei, are to be recognized in this later race ; and 
the only difficulty is that, owing to the imperfection of the geolo¬ 
gical record, very few of these types are revealed until approaching 
1 O. M. Reis, “ Zur Osteologie unci Systematic der Belonorhynckiden und 
Tetragonolepiden,” Geogn. Jahresh. 1891 (1892), p. 157. 
