204 ON THE STRUCTURE OF THE SKULL. 



surface of the cartilage and follows its curves, from its hinder to 

 its anterior extremity, overlapping and folding over the upper 

 edge of the anterior three-fifths of the cartilage. Between the 

 hinder part of E, here cut away, and D, is a space occupied by 

 the levator muscle of the lower jaw. 



The mandibular cartilage extends to the symphysis, and is 

 coated externally, and partially embraced by, the flat bone (Hn), 

 the greater part of the upper edge of which bears teeth. 



On comparing these parts with those of the corresponding 

 apparatus in the embryonic fish (Fig. 72), it becomes clear that 

 the pieces A and B answer to the hyomandibular and symplectic, 

 taken together. Indeed, at first sight, A, supporting as it does 

 the operculum, seems to answer to the hyomandibular, and B to 

 the symplectic itself ; but then it may be suggested that the 

 hyoidean apparatus is attached at the distal end of B, and not 

 between it and A, as it would be if the two corresponded, re- 

 spectively, to the hyomandibular and symplectic. 



The cartilage D obviously answers to the palato-quadrate 

 arch, and that of the lower jaw to Meckel's cartilage. The fact 

 that a levator muscle of the lower jaw passes between E and 

 1) seems to prove the former to correspond with a maxilla; 

 in which case the internal bone would be a sort of palato- 

 pterygoid, similar to that we shall meet with in Lejndosiren. 



The skull of the Sturgeon (Aceipenser), like that of Spatu- 

 laria, is greatly enlarged, posteriorly, by the coalescence with it, 

 and with one another, of six or seven of the anterior vertebra?. 

 In front, it is prolonged into a triangular snout or beak (c, Fig. 

 82 ; a, Fig. 83), the wide base of which is formed by the ant- 

 orbital, or prefrontal, prominences which separate the olfactory 

 chambers from the orbits. Behind the latter are the two great 

 projections (c, Fig. 83) which contain the auditory organs ; and 

 behind these again, and separated from them by a deep lateral 

 fossa, are two wing-like processes (b, Fig. 82), which are directed 

 outwards and obliquely backwards, and proceed, not from the 

 walls of the cranium proper, but from those of the spinal column, 

 where it joins the skull. At this point there is, in the cranio- 

 spinal cartilage of both the Sturgeon and the Sjxdularia, a great 

 dilatation of the neural canal, which is closed above only by a 



