72 PYCNODONTS. 



towards the circumference of the tooth, and consequently at the two 

 extremities of the section, where only the structure above described is 

 visible. Several small twigs pass beyond this plexus into the clear 

 enamel-like outer layer of the tooth, in some parts of which traces were 

 perceptible of a plexus of still more minute tubes, or stri<s, which gra- 

 dually diminished until they escaped the highest magnifying power 

 employed in this examination. (PL 33). The enamel-like layer is 

 clearly a continuation of, and part of the same substance as the rest of 

 the tooth ; its dense and clear texture indicates the extreme subdivi- 

 sion and abundance of the earthy salts : this stratum is of considerable 

 thickness in the Spharodus crassus of Agassiz. 



In the genus Gyrodus the surface of the teeth is furrowed some- 

 times irregularly, sometimes regularly and deeply as in Gyr. rugulosus. 

 (PL 34, figs. 6 and 7). The teeth are present in the intermaxillary, pre- 

 mandibular, palatine and vomerine bones. 



In the Gyrodus umbilicatus each premandibular bone supports four 

 rows of teeth (PL 34, figs. 4 and 5) ; those of the external and third rows 

 are of a transversely oval form, and are larger than those of the second 

 and internal rows, which have a circular contour. The intermaxillary 

 and premandibular teeth are fewer in number, and present an obtuse 

 conical form in the Gyr. circularis. The vomer in this species is 

 covered with five longitudinal rows of teeth ; those of the middle one 

 being the largest as in the Pycnodus. 



In the tooth of a Gyrodus cretaceus, I find that the tendency to the 

 structure of the dense ivory of the teeth of the higher Vertebrata, which 

 is obvious in the teeth of Spharodus 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 vertical sec- 

 tion 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, having a diameter of 

 jTiAh. of a line at their origin, radiating in a direct line, but with a 

 minute and regularly undulating course, and a gradually diminishing 

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

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



