PISCES. 413 



The chondrocranium is always well-developed. It is massive in the 

 Sturgeon and some Teleostei, e.g. Salmon, and its prae-nasal region is 

 occasionally of great size, e. g. in some* RayS) as Pristis, the Sturgeons, and 

 a few Teleostei. It is interrupted by superior fontanelles in Amia ; by 

 lateral and basal fontanelles as well in Polypterus and Lepidosteus (Ganoidet). 

 There are neither investing bones nor intrinsic centres of ossification in 

 Elasmobranchii and Holocephali. In the latter the palato-pterygo-quadrate 

 cartilage is continuous with the cranium ; among the former it articulates 

 with it in Hexanchus and Heptanchiis (Notidanidae\ whilst in others it 

 is attached by ligament and by the hyomandibular 1 . A spiracular cartilage 

 is sometimes found in the edge of the ligament anterior to the spiracle 

 (infra). The lower jaw is a simple rod of cartilage articulating with the 

 palato-pterygo-quadrate and connected to the cranium by the hyomandibular. 

 In other Fish both intrinsic centres of ossification and investing bones are 

 found, and, taking a Teleostean (p. 91) as a standard, the following points 

 may be noted. The jaw-apparatus of Acipenser is only connected to the 

 cranium by a hyomandibular and a symplectic cartilage, both partly 

 ossified: the metapterygoid region of Lepidosteus has an articulation with 

 the base of the skull ; and the palato-pterygo-quadrate is continuous with 

 the cranium in Dipnoi, forming also the sole support of the mandible. 

 Acipenser possesses only a pro-otic, ali- and orbito-sphenoid and an 

 ecto-ethmoid ossification in the cranium, and the mandibular cartilage is 

 unossified as it is also in Dipnoi. Among the bony Ganoidei there is no 

 supra-occipital, no pterotic : in Polyptenis no epiotic distinct from the 

 opisthotic. The prae-pituitary region contains in Amia two ali-, two 

 orbito-sphenoids and ecto-ethmoids : in Lepidosteus two ali-sphenoids and 

 a single bone, perhaps formed by coalesced ecto-ethmoids : in Polypterus 

 two sphenoidal bones and ecto-ethmoids. The palatine and pterygoid 

 bones of Lepidosteus are simply investing bones, as in many Amphibia. 

 Polypterus has no symplectic. The lower jaw has more than one articular 

 ossification, and in Amia a mento-meckelian. The Dipnoihave exoccipitals 

 and palato-pterygoid ossifications. As to the investing bones of the skull 

 there are very many variations. Acipenser has a large number, among 

 which may be recognised a supra-occipital, paired parietals and frontals, 

 and a huge parasphenoid. There are additional bones, not seen in Teleostei, 

 to be found in bony Ganoidei, such as a squamosal over the pterotic region, 

 a supra-temporal bone in Amia, &c. Protopterus among Dipnoi has a 

 fronto-parietal, a median supra-ethmoid (as in the Salmon), two nasals, and 

 a large supra-orbital on each side ; Ceratodus an unpaired parietal and 

 frontal and a pair of lateral bones, the outer perhaps a prae-opercular. 

 The vomer is double in bony Ganoidei, absent in Dipnoi ; in both the 

 parasphenoid is large and extends backwards beneath the backbone as 



1 This palatal cartilage has an upper portion of the hyoid fused with it in the embryo of Selachoidei. 



