MR. T. W. BRIDGE ON THE OSTEOLOGY OF POLYODON FOLIUM. 
687 
in Acipenser and in other fishes is continued along the median line of the dorsum, and 
in the skull by the dermo-supraoccipital and dermo- ethmoid bones. This series is 
rarely continuous, but is repeatedly interrupted in the trunk by the development of 
the dorsal fins, and in the skull by the interpolation of certain elements ot the 
supero-lateral series. The elements of the first series are usually azygous and mesial, 
as in Acipenser, Amia, and Siluroids, &c. ; but both in the skull and in the trunk they 
may be broken up into distinct pieces, as, for example, in the former when there are 
paired dermo-supraoccipitals and paired dermo-ethmoids, as in P olypterus, Lepi- 
dosteus and mauy extinct Crossopterygian Ganoids, or paired dermo-supraoccipitals 
only, as in the extinct Labyrinthodonta. 
The supero-lateral series is represented in the skull by the parietal and frontal 
bones. As is the case with the elements of the first series, the parietals and frontals 
may, in different crania, be either single and mesial, or paired and lateral in position. 
When paired parietals and frontals co-exist with either paired dermo-ethmoids or 
paired dermo-supraoccipitals, the typical mesial position of the latter is obscured, 
and they appear to form elements of the series to which the former belong, and to be 
homologous with them.* 
The lateral or slime-canal series of scales is represented in the cranium by the post- 
temporal, supra-temporal, dermo-sphenotic, and nasal bones lying above the orbit, and 
by the postorbital, suborbital, lachrymal, and preorbital elements situated below the 
orbit. 
The membrane bones forming the infero-lateral series in the cranium are closely 
related to the mandibular and hyoid arches and to the palato-quadrate arcade, and in 
consequence are much more subdivided than are the elements of the supero-lateral 
series. They include, in a linear series, the supraclavicle, operculum, prseoperculum, 
quadrato -jugal, jugal, maxilla, and preem axilla, forming one group of homologues ; the 
clavicle, suboperculum, and interoperculum form second group ; and the inter- 
clavicles, the branchiostegal rays, jugular plates, and mandibular splints constitute 
a third group of serial homologues. On arranging these elements in accordance with 
the transverse segmentation into visceral arches, we have the supraclavicle, clavicle, 
and interclavicle, in relation with the coraco-scapular arch ; the operculum, sub- 
operculum, branchiostegal rays and jugular plates in relation with the hyoid arch, and 
the preoperculum, interoperculum, angular, supra-angular, dentary, quadrato-jugal, 
jugal, and maxilla, associated with the mandibular arch and its pterygoid outgrowth. 
The medio-ventral series is not so distinct as either of the others ; it is possibly repre- 
sented in the skull by the azygous jugular plates, or more probably by the urohyal 
bones of Teleostei. 
* The bones which Mr. Parker calls paired dermo-supraoccipitals in Callichthifs seem to me to be 
really the first pair of supero-lateral plates, while his median parietal is really the dermo-supraoccipital. 
At all events the latter seems to correspond to the bone which in Clarius and other Siluroids is called by 
that name, and which is also perforated by a median foramen. 
MDCCCLXXVIir. 4 T 
