In some Teleosteans the differentiation of the teeth into biting teeth and crushing teeth is as complete as in Lepidosteus. Thus in the Wrasse (Labrus), the jaws bear conical slightly recurved teeth arranged in one or two rows, with some of the anterior ones much larger than the rest. The bones of the palate are toothless, while both upper and lower pharyngeal bones are paved with knob-like crushing teeth; such pharyngeal teeth occur also in the Carp but are attached only to the lower pharyngeal bone, the jaw bones proper being toothless.
In Dipnoi the arrangement of the teeth is very similar to that in Holocephali. The mandible bears a single pair of grinding teeth attached to the splenials, and a corresponding pair occur on the palato-pterygoids. In front of these there are a pair of small conical vomerine teeth loosely attached to the ethmoid cartilage. The palato-pterygoid teeth of Ceratodus are roughly semicircular in shape with a smooth convex inner border, and an outer border bearing a number of strongly marked ridges. The teeth of the extinct Dipteridae resemble those of Ceratodus but are more complicated.
ENDOSKELETON.
Spinal column[41].
The spinal column of fishes is divisible into only two regions, a caudal region in which the haemal arches or ribs meet one another ventrally, and a precaudal region in which they do not meet.
The various modifications of the spinal column in fishes can be best understood by comparing them with the arrangement in the simplest type known, namely Amphioxus. In Amphioxus the notochord is immediately surrounded by a structureless cuticular layer, the chordal sheath. Outside this is the skeletogenous layer, which in addition to surrounding the notochord and chordal sheath embraces the nerve cord dorsally, and laterally sends out septa forming the myomeres.
The Cartilaginous ganoids[42] Acipenser, Polyodon and Scaphirhynchus are the simplest fishes as regards their spinal column. The notochord remains permanently unconstricted and is enclosed in a chordal sheath, external to which is the skeletogenous layer. In this layer the development of cartilaginous elements has taken place. In connection with each neuromere, or segment as determined by the points of exit of the spinal nerves, there are developed two pairs of ventral cartilages, the ventral arches (basiventralia) and intercalary pieces (interventralia); and at least two pairs of dorsal pieces, the neural arches (basidorsalia) and intercalary pieces (interdorsalia). The lateral parts of the skeletogenous layer do not become converted into cartilage, so there are no traces of vertebral centra. The ventral or haemal arches meet one another ventrally and send out processes to protect the ventral vessels. The neural arches do not meet, but are united by a longitudinal elastic band.
In Cartilaginous ganoids the only indications of metameric segmentation are found in the neural and haemal arches. The case is somewhat similar with the Holocephali and Dipnoi.
In the Holocephali the notochord grows persistently throughout life, and is of uniform diameter throughout the whole body except in the cervical region and in the gradually tapering tail. The chordal sheath is very thick and includes a well-marked zone of calcification which separates an outer zone of hyaline cartilage from an inner zone. There are also a number of cartilaginous pieces derived from the skeletogenous layer which are arranged in two series, a dorsal series forming the neural arches and a ventral series forming the haemal arches. These do not, except in the cervical region, meet one another laterally round the notochord and form centra. To each neuromere there occur a pair of basidorsals, a pair of interdorsals, and one or two supradorsals. In the tail the arrangement is irregular.
In the Dipnoi as in the Holocephali the notochord grows persistently and uniformly, and the chordal sheath is thick and cartilaginous though there are no metamerically arranged centra. The neural and haemal arches and spines are cartilaginous and interbasalia (intercalary pieces) are present. The basidorsalia and basiventralia do not in Ceratodus meet round the notochord and enclose it except in the anterior part of the cervical and posterior part of the caudal region.