180 
MR. W. K. PARKER ON THE STRUCTURE AND 
process” shows in Polyodon a nearer relationship to the larval Frog; the absence of a 
metapterygoicl plate brings it nearer to the Sharks, and the three pairs of ectosteal 
centres to the Holostean Ganoids. (See Bridge, Phil. Trans., 1878, Plates 55-57.) 
Comparsion with the Selachians. 
The development of the basis-cranh and cranial walls in Acipenser is very similar 
to what is seen in the Selachians; and the after modifications are essentially alike, 
except that in the Sturgeon the cartilage is very massive, and the occipito-cervical 
articulation is not formed. The separation of a large symplectic, and a lesser inter- 
liyal segment, the complex metapterygoid plate, and the partial ossification of the 
visceral arches, are all modifications that separate this from the Selachian types. 
Comparison with the Holostean Ganoids. 
Most of the peculiarities which distinguish the skull in that group from that of a 
Sturgeon are so many steps in the direction of the Teleostean skull. To say nothing 
of the lighter build of the chondrocranium, the numerous ectosteal patches in the 
skull wall, the intenser ossification of the visceral arches, and the absence of a distinct 
symplectic cartilage (it may exist as a separate bony centre, as in the Teleosteans), all 
these things show how far the Holostei have become specialized beyond the chon- 
drosteous Acipenser. An important modification occurs early in Lepidosteiis, namely, 
that the palato-quadrate is continuous, at first, with the trabecula; the intertrabecula 
also appears earlier. I shall compare the skull of the Ganoids, generally, with that of 
the Teleostei, in my next paper, which will treat of the skull of Lepidosteiis. 
