930 



PISCES. 



come elongated by the addition of successively 

 calcified portions of their pulp to their basal 

 or posterior extremities; the opposite end exerts 

 a proportional pressure against the circum- 

 ference of the foramen, and causing its ab- 

 sorption begins to protrude. The tuberculate 

 crown of the denticle is exposed about the 

 time when its sides become anchylosed to those 

 of the previously protruded row. Thus, from 

 the close apposition of the protruding den- 

 ticles, the whole of the outer parietes of their 

 common alveolar cavity subjected to the sti- 

 mulus of their pressure is finally removed, and 

 is replaced by the pavement of mutually an- 

 chylosed teeth (fig. 515, ). 



In the Diodom and Tet radons the structure 

 of the teeth is equally peculiar, but of a very 

 different character from what has been described 

 above. In the former genus each jaw is fur- 

 nished with a double compound tooth adapted 

 to crush and bruise the food, the structure of 

 which at once reminds the anatomist of the> 

 molar teeth of the elephant. Each tooth 

 (fig- 517) consists of numerous lamina; super- 

 imposed upon each other, the upper ones 

 being the oldest and most worn, while the 

 lower ones are the largest and most recently 

 formed. In consequence of this arrangement 



Jig. 517. 



the Tet radons a somewhat similar structure of 

 the dental organs is met with. 



The rostral teeth of the Saw-fishes, Pristls, 

 are quite unique among the whole race of 

 Fishes from the singular position which they 

 occupy, as will be perceived by the following 

 account of this strange apparatus extracted 

 from Professor Owen's elaborate treatise. 



" The maxillary teeth of the Saw-fish, which 

 is an active and predatory Shark, are, notwith- 

 standing its habits, extremely small, simple, 

 obtuse, and wholly inadequate to destroy and 

 secure the prey requisite for its subsistence. 

 But this seemingly imperfect armature of the 

 mouth is compensated for by the develope- 

 ment from the anterior part of the head of a 

 very singular and formidable weapon provided 

 with strong lateral teeth, and which from its 

 resemblance to a saw has given rise to the ver- 

 nacular of ' Saw-fish,' applied to the present 

 species of Shark." 



In most of the Plagiostomes, but especially 

 in the group of Squaloids, a conical projection 

 or cutwater is continued from the fore -part of 

 the head, and its framework is composed of 

 peculiar and superadded cartilages articulated 

 to the anterior extremities of the frontal, nasal, 

 and vomerine bones. These rostral cartilages 

 in the Saw-fish ( Print is antiquorum) are 

 blended into a horizontally flattened plate, 

 which is produced to a length equalling one- 

 third that of the entire fish : this process is 

 more completely ossified than any other part 



Fig. 518. 



Section of the lower jaw of the Globe-fish ( Diodon), 

 shewing the structure of its teeth. 



a, circumference of jaw ; b, grinding surface of 

 tooth within the mouth. (After Owen.) 



the exposed surface of the tooth that projects 

 into the mouth (b) presents a grooved appearance, 

 being formed of the edges of the contiguous 

 lamellae that are situated towards the upper 

 part of the tooth. As these superior laminae 

 are worn away, it is evident that they are con- 

 tinually replaced by the advancement upwards 

 of the inferior plates, so that the tooth is kept 

 constantly efficient for service. The circum- 

 ference of the jaw () is formed of super- 

 imposed plates which grow from below in a 

 precisely similar manner; but between these 

 and the posterior laminated tooth a very diffe- 

 rent structure is interposed, which is revealed 

 by the microscope to consist of a series of 

 narrow flattened denticles lying horizontally and 

 at right angles to the anterior surface of the 

 jaw. These denticles are developed in a cavity 

 between the outer and inner walls of the jaw, 

 the floor of which is formed by a thin cribri- 

 form osseous plate separating the cavity con- 

 taining the teeth from the wide vascular canal 

 which runs in the substance of the jaw. In 



Rostrum of Saw-fish ( Pristis antiquorum), shewing 

 the marginal teeth. 



