REMAINS OF EEPTILES AND FISHES. 93 



outer margins, are three other bones, which form the lateral bor- 

 ders of the cranium. In all there are five bones on each side of 

 the central occipital and posterior part of the parietals. Only a 

 small portion of the left parietal is preserved; but enough is pre- 

 sent to show that this pair of bones are elongated, being widest 

 apparently a little behind their centre, and having their posterior 

 margins slightly divergent to receive the anterior angle of the 

 central occipital. 



The surface of the bones is not ornamented with " waved and 

 bent lines," as those of Dipterus are described to be by Miller 

 [ibid p. 61), but is minutely granulated and punctate, similar to 

 that of the opercles described in the paper on Ctenodus already 

 referred to, and here and there are indications of the radial bone 

 structure beneath. 



The original estimate of the width of this head was nine in- 

 ches. It is now evident that it really was 8|- inches across the 

 occipital region, without taking into account the fragmentary 

 bone, probably a portion of an operculum. Were this added to 

 the above measurement, the width would be ten inches. 



The external characters of the palatal plates of the various 

 species of Ctenodus were described in the paper on that genus 

 mentioned above. Nothing, however, was said of the internal 

 structure, such matters of detail having been reserved for some 

 future occasion. But it is now perhaps desirable to give some 

 account of the microscopical character of these peculiar dental 

 plates. 



In sections made across the transverse ridges that cover the 

 whole surface of the plates, a very beautiful structure is presented 

 to view. The entire substance is found to be composed of a 

 minute reticulation of bone-like matter, the meshes or medullary 

 canals being large and much complicated. The ridges stand up 

 from the surface in the form of conical tooth-like processes ; and 

 the reticulated matter of which they are composed is perfectly 

 continuous with that of the plate or base ; but the meshes or 

 medullary canals in them are a little elongated, and the surface 

 is protected by a compact, rather thin layer, which is only 

 distinguishable from the rest of the tissue bj- its density and 



