96 DESCRIPTIONS OF PREPARATIONS. 



Holocephali, and Dipnoi, which possess a pelvis. The fin-rays are articulated 

 to the posterior and external faces of this double bone. The first ray in 

 each fin is entire, the remaining five are soft. 



The following additional points may be noted relative to the skull. A series 

 of suborbital bones lies beneath the eye of which the most anterior is exceedingly 

 large. These bones are removed in this specimen. In some Teleostei, e.g. the 

 Trout, there are similar bones above the eye. The optic nerve passes out of the 

 cranium between the arms of the basisphenoid. The bone termed alisphenoid is 

 continued forwards for a short distance by membrane which is pierced by the 

 olfactory nerve. The two nerves separated by the interorbital septum traverse the 

 back of the orbit and pierce the ectoethmoids on the way to the olfactory mucous 

 membrane. An ' ocular' canal which lodges the recti muscles of the eyes passes 

 backwards into the forepart of the basi-occipital, beneath the pro-one and above the 

 parasphenoid. The inner wall of the ear-capsule is replaced by thin membrane 

 which is easily injured in dissection, and the vertical semi-circular canals then 

 appear to lie in the cranial cavity. 



The skulls of Teleostei in general agree closely with that of the Perch. The 

 size of the cartilaginous cranium varies, as may be seen on comparison of a Salmon 

 with the Perch. The amount of persistent cartilage as compared with bone also 

 varies. Anchylosis of the bones may take place. In Cyprinoid and Siluroid fish 

 there is no inter-orbital septal plate, but there are orbito- and pre-sphenoid ossifica- 

 tions in this region. In the Pike the mesethmoidal cartilage is partially ossified by 

 two pairs of bones instead of a median one, and in the Salmon this region is not 

 ossified but is covered by a supra-ethmoid bony plate. The Pike has a small 

 supra-orbital bone. Praemaxillae are absent in Muraenoids. In some Teleostei, 

 e.g. Pike, a small bone placed distally on the posterior margin of the maxilla 

 appears to represent the jugal. The metapterygoid is absent in the Siluroid Clarias 

 capensis. And in Siluroids generally the sub-opercular is absent. In Cyprinoids the 

 fifth branchial arch is strongly bowed, and carries prominent teeth which work 

 against the horny basi-occipital tooth. The group of Pharyngognathi is so named 

 from the fact that the fifth pair of arches is fused into a single dentigerous plate. 



When the lower jaw is connected to the cranium solely by a hyomandibular 

 element derived from the hyoid arch as it is in Teleostei, in Ganoidei except Lepi- 

 dosteus, in the majority of Elasmobranchii, it is said to be hyostylic. When it is 

 connected not only by a hyoidean element but also by a quadrate, as in Lepidosteus, 

 or by a palatoquadrate, as in Cestration among Elasmobranchii and probably in 

 Holocephali, it is said to be amphistylic ; and when it is connected by a quadrate 

 element alone as in Amphibia and Sauropsida, it is termed autostylic. In Mam- 

 malia the dentary element articulates with the squamosal, the articular portion of 

 the lower jaw and the quadrate having been converted into ear-bones, i. e. malleus 

 and incus respectively. 



The azygos system of fins appears in Teleostei as in other fish as a continuous 

 fold of skin supported by embryonic fin-rays which are afterwards replaced by per- 

 manent fin-rays. Such a continuous fold extending from the back of the head 

 round the tip of the tail to the anus persists in Blennies, Eels, Congers, Soles, 

 Ophididae, &c., among Teleostei, and in Dipnoi* But this condition in the 



