688 
MR. T. W. BRIDGE ON THE OSTEOLOGY OF POLYODON FOLIUM. 
This slightly modified schema differs from the one suggested by Professor Parker 
only in the recognition of a medio-dorsal and a medio-ventral series, and the consequent 
alteration in the elements which are to constitute homologous series. Thus I do not 
consider the dermo-supraoccipital, the parietals and frontals, and the dermo-ethmoid as 
serial homologues, but prefer to regard the first and the last as forming one series of 
homologues — the medio-dorsal, while the two other bones form a second series — the 
supero-lateral. As will be seen shortly, this arrangement is justified by the disposi- 
tion of the elements of the cranial dermal armature in Acipenser. 
W e are now in a position to endeavour to ascertain how far the cranial parostoses of 
Polyodon may be expressed in terms of this schema. 
Huxley* refers as follows to the posterior cranial splints and the fontanelles 
which they enclose : “In the dorsal region it presents an anterior and posterior pair 
of perichondrial ossifications, separated by oblong laminae from lateral bony plates 
of the same character, but the homology of these with those in the roof of the 
Teleostean skull is not to my mind satisfactorily demonstrated.” The “anterior ^and 
posterior pairs of ossifications” referred to are those marked b l and b 2 , and the “lateral 
bony plates of the same character” are c 1 and c 3 . According to Owen! the two first- 
mentioned pairs of bones are the parietals and frontals respectively, and the two latter 
the mastoid and postfrontal elements. 
At first sight it seemed not impossible that the splints b l were the homologues of the 
paired dermo-supraoccipitals of Polypterus, Lepidosteus, and the Labyrinthodonta, but 
as they evidently belong to the same supero-lateral series as the paired splints, b' 2 , which 
are separated by the medio-dorsal element a 1 , I am inclined to agree with Owen, and 
regard them as corresponding to the parietals of other vertebrata, mesially united owing 
to the suppression of the proper median bone — the dermo-supraoccipital. It will be 
noticed that the splints (6 3 ) forming the third pair of supero-lateral elements are also 
in contact along the median line of the skull, in front of the azygous element, a 1 , just 
as the parietals are mesially opposed behind it, thus conforming to the alternating 
arrangement of the medio-dorsal and supero-lateral splints of the roof of the rostrum. 
The next pair of supero-lateral splints (b 2 ) are clearly the equivalents of the frontals, 
while a 1 , the median bone which separates them, is a dermo-ethmoid. Three serial 
homologues of the last-mentioned bone (a 3 , a 3 , a 4 ,) continue the median series to the 
end of the rostrum. In a similar manner the parietals and frontals are serially 
repeated by their paired homologues b s , />', and b 5 . The separation of the elements of 
the dermo-ethmoid series by the interpolation and mesial apposition of the paired 
supero-lateral splints has already been pointed out. 
The bone c 1 , the most posterior element of the lateral series, serves to attach the 
supraclavicle to the skull, and might, therefore, be regarded as a post-temporal splint, 
especially as there is no other bone which can possibly represent that element. Its 
* ‘ Lectures on the Vertebrate Skull,’ p. 203. 
f 4 Osteological Catalogue of the Museum of the College of Surgeons,’ vol. i. p. 83. 
