308 PROCEEDINGS OF THE GEOLOGICAL SOIETT. [Feb. 21, 



layer, in whose upper portion it breaks up into a series of fine taper- 

 ing, anastomotic tubules, which seem to open on the outer surface. 

 The bone on which this dental plate rests presents the usual lami- 

 nated structure in its deeper portions. Towards its alveolar surface 

 the lacunae enlarge, and become more spherical: medullary canals 

 make their appearance, and increase both in number and size up- 

 wards, the bone occupying a progressively smaller area as compared 

 with that of the canals. Into each of these canals seldom more than 

 one of those of the dental plate open, the intervening dentine resting 

 on, but not being continuous with, the osseous septa. The same modi- 

 fication of structure as may be traced from above downwards in the 

 jaw, is found to occur towards either alveolar margin : the medullary 

 canals disappear, the lacunae become fusiform, and at length merely 

 separate laminae of compact bone, which shghtly overlaps the lower 

 lateral angle of the base of the plate. 



The osseous matrix is thin in proportion to the height of the tooth. 

 The surface vertically opposite the intertubercular groove is very 

 irregular, while the imperfect section of the maxillary vertical plate 

 shows that it did not increase in thickness inferiorly. The maxilla 

 was therefore L-shaped. It is evident that a dental plate so con- 

 structed is unequal to the crushing indicated by the tubercular teeth. 

 The palato-vomerine plates are also weak for the work for which 

 they are adapted. It can scarcely be held that the thinness of the 

 jaws is due to the loss of an articular plate in the mandible, or of 

 any part of the maxilla. It seems more probable that, as the incom- 

 pleteness of the basicranial structures points to their cartilaginous 

 condition during life, so the three tooth-bearing bones are incomplete 

 by the loss of the cartilaginous cushions on which they rested. That 

 the premaxilla also received some similar support, is probable from 

 its fracture showing a thick outer case of bone, inclosing an interior 

 hoUow. 



This whole masticatory apparatus is very complex in its morpho- 

 logical relation. The jaw-plates bear some resemblance to those of 

 Chimaeroid fishes, especially Edapliodon, in which the tooth is 

 socketed in a jaw of ordinary osseous tissue ; but the medullary 

 canals of that genus are much larger, and become closed superiorly 

 by calcification. In Cestracion the calcigerous tubes come off as in 

 Am^hicentrum, but are lost in the superficial dense layer. Prom 

 Pycnodonts, the socketing of the teeth of Am^phicentrum, the thin- 

 ness of the enamel layer, the presence of a cement layer, and of 

 medullary canals are sufiicient distinctions. To Psammodus and 

 Ptychodus the resemblance of the dental plates is closer. In both 

 these genera, however, the canals are smaller, more numerous, 

 branch more freely, and terminate by breaking up into tufts of cal- 

 cigerous tubes, whereas in Amphicentrum no such breaking up 

 occurs. In the latter, again, the ossified dentine is separated from 

 the osseous matrix ; in the former the two structures are continuous. 

 The pharyngeal teeth of Sargus and the teeth of Sargodon also offer, 

 in their mode of attachment and in their minute structure, resem- 

 blances to the genus under description, which are further made in- 



