1866.] YOUNG PLATYSOMTJS. 309 



teresting by the jaw-teeth of the living genera having some common 

 characters with those of Pycnodus. The pharyngeal denticles are 

 implanted in a matrix of widely cancellated bone, which becomes 

 dense and lunular towards its alveolar borders. The ossified dentine 

 is distinct from the matrix, and is not divided into enamel and den- 

 tine, the outer layer being simply a portion of the tooth-substance, 

 in which the calcigerous tubes form a fine reticulation. If several 

 of these denticles were continuous in place of contiguous, the dental 

 plates of Amphicentrum would be reproduced. 



Lying in the wide gape of the large specimen figured are two nar- 

 row plates, the one in cast only, the other the plate itself (PI. XX. 

 fig. 7). On the latter are three longitudinal rows of alternate 

 teeth, those of the lateral rows not opposite each other, but alter- 

 nate, hence giving the transverse series a spiral aspect. The plate 

 is not entire, its limits, therefore, are indeterminable. It is slightly 

 curved anteriorly, the straight posterior part containing the larger 

 teeth. These are longitudinally oval, evenly curved in both axes, 

 the smaller teeth are spherical. Their surface is studded with fine 

 granular pits, as in Psammodus. Under a lens the tubercles are 

 structurally identical, and seemingly continuous with the interven- 

 ing plate, than which they have a more vitreous aspect, as if a 

 denser enameUoid layer invested their surface. They are, in fact, 

 to outward appearance the counterparts of the maxillary and man- 

 dibular apparatus: it is to be regretted that of this structure no 

 microscopic section can be obtained. The plate, whence the tubercles 

 spring, is very thin, |^th of an inch in thickness ; the under surface 

 is set with fine, distant, conical pits, to judge by the cast exposed 

 on the shale. The resemblance to the jaws is therefore considerable, 

 in the thinness of the osseous layer and irregularity of its under 

 surface. The anatomical value of this plate is not quite clear. As 

 no bone corresponding to the edentulous plate in Platysomus is found, 

 these two curved dental plates may, with great probability, be taken 

 to represent the palato-vom*rine mass. As there seems no reason 

 for considering the curvature accidental, they differ from the same 

 parts in Pycnodus, in that their inner margins were not in contact ; 

 the arch of the palate was, therefore, incomplete, or closed by mem- 

 branes only ; at least there is at present no evidence of a central 

 series of crushing teeth, or of an unarmed bony plate closing the 

 gap. A specimen in Mr. Ward's possession shows that the transverse 

 breadth of the head is in agreement with the supposition that the 

 two rami were apart. 



The suspensorium is very strong, descending nearly vertically from 

 the squamosal. The close union of the bones forbids any attempt to say 

 what were the components of the bar. A pterygoid plate is attached 

 to it anteriorly and superiorly ; but the other constituents of the 

 palato-pterygoid arch are concealed beneath the maxillary vertical 

 plate. There is an indication on one specimen, of facial plates over- 

 lying this vertical portion, as in Tetragonolepis and its allies, parts 

 in fact of the so-called outer orbital ring ; but, if the surmise is 

 correct, they are in less close relation to the true suborbital ring-, 



