epidermis 



two generations of 

 horny teeth 



highly vacuolated cells 

 position of third horny cap 



dermis 



Figure 8-51. Section through o lamprey tooth. 



urn cells 

 dermal papilla 



epidermis 



cartiloge'' 



Figure 8-52. Section through o dental plate tooth of Myxme. (After 

 Marinelli and Strenger, 1956, and Lison, 1954) 



not resemble those on the plates of known agnaths, the pos- 

 sibility of their being gnathostome teeth should be recog- 

 nized. 



The arthrodires generally lack teeth on the jaw margins, 

 but some of the smaller forms have them (Figure 8-54). In 

 Alopacanlhus, the teeth are fused to the acellular jaw bones 

 and have an outer enamel, or modified dentine layer, and 

 an orthodentine core. 



Chondrichthyes 



The teeth of sharks vary from simple conical points to 

 multituberculate elements or broad, flat crushing plates. 

 The general structure is that of the placoid scale. In the 

 shark Lamna. the pulp cavity of the tooth is largely filled 

 with an osteodentine that also forms the basal plate (see Fig- 

 ure 8-23 B). In Myliobatis, the flattened, crushing teeth have 

 a tubular dentine in their interior (Figure 8-55). 



The development of the teeth in sharks is much like that 



Figure 8-53. Lower Ordovician denticles identified as Palaeodus sp?, 

 A and Poloeodus brev.s, B. (A after Jarvik, 1 960; B after Peyer, 1 937) 



living cyclostomes, horny teeth are present in the mouth 

 funnel and on the rasping organ. These are formed of kera- 

 tinized epithelial cells (Figure 8-51, 8-52). 



Early gnathostome fishes 



In the gnathostomes the teeth are typically conical in 

 shape and formed of the same materials as the skin denticles. 

 The earliest vertebrate remains, from the Lower Ordo- 

 vician of Esthonia, are described as dermal denticles of two 

 genera, Palaeodus and Archodus. The structure seen in the 

 case o{ Palaeodus (Figure 8-53) is that of a tooth. The der- 

 mal denticles of fossil agnaths are low, rounded tubercles, 

 ridges, or hooked points like the tip of a placoid scale. Iden- 

 tification of the Palaeodus denticles as belonging to the der- 

 mal armor is based on the supposition that agnaths precede 

 gnathostomes in time and that gnathostomes are unknown 

 from deposits older than Silurian. Since these denticles do 



Sharpey'sfibers- If 



mandible of acellular bone 



Figure 8-54. Stereodiogrom of port of lower jaw of an acanthodian. 



Atopocanthus sp. (After Orvig, 1957) 



246 • THE SKIN AND ITS DERIVATIVES 



