38 PROF. T. H. HUXLEY ON CERATODUS FORSTERI. [Jan. 4, 



one specimen, the hinder end of each inner lateral bone is completed 

 by a distinct ossification (C). There is also a separate ossification 

 {E) on the left side, on what corresponds with the front part of 

 the outer lateral bone on the right side. Doubtless these bones are 

 subject to much individual variation. 



The fibrous band which extends, below the eye, between the ant- 

 orbital process and the ventral end of the suspensorium contains 

 three suborbital bones. Theanteriorof these, trihedral, is connected by 

 its broad base with the antorbital process ; the middle bone is slender 

 and elongated ; the posterior is broad, flattened from side to side, and 

 its hinder edge is fixed by ligamentous fibres to the outer face of the 

 suspensorium and of the squamosal. 



The basal bone of Dr. Giinther is the parasphonoid. It extends 

 backwards, beyond the limits of the proper cranium, into the region 

 of the vertebral column, to a point just beyond the attachment of the 

 third pair of ribs. But there is at least one vertebra in front of that 

 which bears the first pair of ribs. In Ceratodus, as in the Sturgeon 

 and other Ganoids, several anterior vertebrae have coalesced with one 

 another and with the skull ; or, probably, it would be more accurate 

 to say that the investing mass of the notochord has not become 

 differentiated into vertebrae for this extent. Nevertheless the pos- 

 terior boundary of the skull can be strictly defined by the interspace 

 between the exit of the pneumogastric and that of the next following, 

 or first spinal, nerve. 



It is to the outer surface of this interspace that the anterior edge 

 of the "suprascapular" element of the pectoral arch Is fixed by 

 strong ligamentous fibres (fig. 2). Just in front of the boundary 

 between the skull and the vertebral column, and therefore in the side 

 walls of the former, there lies, deep in the substance of the cartilage, 

 a hollow cone of bone (E.O) It is wider above and externally than 

 below and internally, where its end lies above the notochord. This 

 appears to be an exoccipital ossifi'cation, such as is to be found in 

 greater state of development in Lepidosiren, Polypterus, and Meno- 

 branchus. 



The skull of Ceratodus is, as might be expected (and as Hy. 

 Giinther has pointed out), extremely similar to that of Lepidosiren. 

 In fact, beyond differences in the proportions of its various parts, 

 the more extensive fenestration of the roof of the olfactory capsules 

 in Lepidosiren, and the absence, so far as my investigations have yet 

 gone, of the hyomandibular cartilage in the latter genus, the carti- 

 laginous elements of the skull are the same in the two cases. 



As to the superadded bones, the parasphenoid, the rudimentary 

 vomers, and the pterygopalatine plates correspond in the two genera. 

 The exocoipitals are much larger in Lepidosiren. The descending 

 process or prseopercular part of the squamosal is best developed in 

 Lepidosiren, whilst its dorsal part (proper squamosal) is larger in 

 Ceratodus. 



In both, there are two opercular bones, an operculum and an inter- 

 operculum ; and in Lepidosiren, as in Ceratodus, there are cartilagi- 

 nous plates attached to the inner faces of these bones. 



