TRANSACTIONS OF THE SECTIONS. 143 



genus, e. g. Lepid. Filtoni, correspond so closelj'^ in size and form with 

 those of the preceding genus, (Sphcerodus,) as not to be distinguishable 

 from them but by a comparison of their microscopic structure. They 

 are composed of fasciculate tubes continued directly from the cells of 

 the osseous base, radiating, with a direction vertical to the surface of 

 the tooth, and giving off branches, at an acute angle, from their very 

 commencement : thus far the general character of the texture of the 

 tooth is the same ; but the fine branches into which the fasciculate 

 tubes resolve themselves, diverge at a much more open angle from the 

 main trunk, are spread out more widely, have a more wavy course, and 

 present the appearance of corn beaten down with heavy rain. These 

 live terminal branches are inextricably interwoven, and present the ap- 

 pearance of numerous anastomoses, but do not form so dense a structure 

 as to intercept the light, as is the case in the teeth of Sphcerodus. 



Gyrodus. — In the pharyngeal teeth of this genus, the tendency to 

 the structure of the dense ivory of the teeth of the higher vertebrata, 

 which is obvious in the teeth of Sphcerodus and Lepidotus, is carried 

 on to a close correspondence. The base of the tooth is excavated by 

 a large and simple pulp-cavity, presenting a quadrate figure in a ver- 

 tical section of the tooth ; this cavity is immediately continuous with 

 the large cells and reticulate canals of the bony base. The body of 

 the tooth consists of close-set minute calcigerous tubes, haying a dia- 

 meter of 7tH)th of a line at their origin, radiating in a direct line, but 

 with a minute and regularly undulating course, and a gradually dimi- 

 nishing diameter to the superficies : the lateral tubes pass horizontally, 

 those continued from the summit of the pulp-cavity vertically, to the 

 grinding surface. They give off very regular, but extremely minute 

 branches, which are lost in the clear and dense enamel-like superficial 

 layer of the tooth. 



Barbel. Pharyngeal tooth. — In this tooth the structure character- 

 istic of the ivory of the simple mammalian tooth is beautifully dis- 

 played. The cavity of the pulp is single, elongated and narrow, and 

 the tubes radiate to the surface of the tooth at right angles to that sur- 

 face, and chiefly, therefore at right angles to the axis of the tooth. 

 The tubes are minute and numerous, beautifully and regularly undu- 

 lating, seldom dividing, and then dichotomously, each branch proceed- 

 ing nearly in the direction of the trunk. A detached fossil pharyngeal 

 tooth of this kind would be distinguishable from a mammalian carni- 

 vorous tooth of similar form by the circumstance, that in the tooth of 

 the fish the pulp-cavity becomes directly continuous with the coarse cells 

 and medullary canals of the bone with which it is anchylosed ; the base 

 of the tooth is not diminished to a fang, and the calcigerous tubes are 

 larger and more irregular the closer they are to the base of the tooth. 



The large conical carnivorous teeth of the extinct genera Holopity- 

 chns and Megalichthys present a similar grade of structure to that of 

 the pharyngeal tooth above described. The whole body of the tooth is 

 here composed of minute close-set calcigerous tubes, having a diameter 

 of x5\t(7^'^ ^^ ^ ''16 "1 diameter, with interspaces of nearly twice that 

 diameter. The calcigerous tubes have a minutely undulated course, 



