ment in this group, a phenomenon referred to as lyodonti. 



The holocephalan is unHl<e the shark in that it has tooth 

 plates associated with the jaws, two pairs above and one 

 below. Because of their holostylic jaw suspension, the chi- 

 maerids have been related to the late Palaeozoic bradyodont 

 sharks. The latter have simple, pluriserial (many rows) 

 teeth with broad, fiat crowns. The rows of teeth form 

 a broad, solid band along the jaw margin. There is some 

 heterodonti, i.e. difference in size and shape, from front to 

 rear in the band. The teeth are not replaced, a condition 

 described by the term statodont!. They have an outer 

 pallial dentine cover, and the crown is supported by tubu- 

 lar dentine on an osteodentine base, as in the living ray, 

 Myliobatis (Figure 8-55). 



The teeth of Hydrolagus are quite unlike those of the 

 bradyodont, since they are fused into plates with a narrow 

 cutting edge. These plates grow continually but are not 

 shed. Examination of a mandibular plate reveals the outer 

 and inner surfaces covered by pallial dentine. Inside the 

 outer shell is a layer of spongy dentine whose channels tend 

 to be vertically disposed, and in this osteodentine there are 

 columns of ovoid spaces filled with calcified material. These 

 outer layers, along with the supporting columns of calcified 

 balls, form the outer beveled edge of the plate. This mar- 

 gin has an anterior and a middle cusp. Inside this is a layer 

 of spongy dentine in which the cavities tend to run ante- 

 rior-posteriorly. Vertical ridges of spongy osteodentine lie 

 medial to this and to the anterior and middle cusps. These 

 ridges are strengthened by columns of calcified balls. 



In this tooth plate there is a developmental sequence 

 from root to crown. The root is quite spongy and fibrous 

 with wide anastomosing channels. The sacs of the columns 

 are filled with a soft material, predentine. Near the crown, 

 the spaces are largely filled with osteodentine and all parts 

 are well calcified. The middle spongy layer is the softest part 

 of the plate and forms a vale between the outer and inner 

 layers. This middle layer is lacking in the anterior cusp where 

 the columns of calcified balls lies about midway between 

 the outer and inner shells. 



Actinopterygians 



The teeth of actinopterygians are usually cones. In the 

 palaeoniscoids these have a hard, shiny, outer cover of 

 enamel over pallial dentine and a large pulp cavity. Gen- 

 erally, they are fused to the jaw margin. Again there is the 

 question whether the outer cover is enamel. In some fishes 

 a prismatic structure comparable to enamel is achieved. 

 The outer cover has been identified as vitrodentine or 

 hyodentine and is presumed to represent a modification of 

 the surface of the dentine through the agency of the amelo- 

 blasts of the epidermal enamel organ. The dentine of some 

 fishes is peculiar and is identified as vasodentine. Capillary 

 loops permeate this tissue which lacks dentinal canals (Fig- 

 ure 8-57). 



Tooth form is variable in these fishes, as in other groups. 

 There are large fangs or flat-surfaced crushing teeth. The 

 bases of the teeth in Lepisosteus (Figure 8-58) show vertical 

 ridges and in section are observed to be of a simple, 

 labyrinthine type. The teeth in this group are continually 

 replaced by new teeth forming medially to the old. 



Choanate fishes 



The crossopterygian fishes have rather large conical 

 teeth, whose bases are strengthened by folding of the den- 

 tinal walls (plicidentine) (Figure 8-59, 8-60). This labyrin- 

 thodont condition is observed in the garfish, an actinopter- 

 ygian, in the early amphibians, and in some reptiles. 



The labyrinthodont condition is not observed in the 

 primitive dipnoan Dipterus; however, in this genus, marginal 

 teeth are lacking and tooth plates are formed on the 

 pterygoids and prearticular. The tooth in this genus is 

 strengthened by filling the pulp cavity with osteodentine 

 (Figure 8-23 B). 



In the living lungfishes, the tooth plates are formed by 

 the basal fusion of many teeth. The teeth are joined together 

 by an enamel-like material and are filled interiorly with 

 tubulodentine. 



Amphibians 



In the primitive amphibians, the labyrinthodont type of 

 tooth is well developed (Figure 8-61): the teeth are often 

 large and set either in shallow pits or in an alveolar groove. 

 The point of the tooth is simple, but toward the base the 

 dentine becomes infolded to form a very complicated pat- 

 tern as seen in cross section. The pulp cavity is constricted 

 by this infolding process. 



tubulodentine 



connective tissue 



pongy bone 



. jaw bone 



Figure 8-57. Section through tooth and jaw of Mer/ucius vu/gons a 

 teieost. (After Peyer, 1937) 



248 • THE SKIN AND ITS DERIVATIVES 



