170 SAURICHTHYS. 



merits drawn from the rest of its organization, (1) that the Lepidosiren 

 is in every essential point a member of the class of fishes. 



SAURICHTHYS. 



65. I terminate the description of the dental system of fishes with 

 an account of the microscopic structure of the teeth of two extinct 

 species of different genera, indicated as yet only by these detached 

 organs, and respecting which, doubts have been entertained as to 

 whether they be actually referable to the class of fishes or of reptiles. 



To these doubts, however,^ in regard to the first genus, Sau- 

 richthys,{2) the microscopic test satisfactorily puts an end ; but with 

 respect to the other genus, Dendrodus, the modification of the dental 

 tissue approaches so closely to a new and singular condition which 

 will be shown to characterize the structure of the teeth of an extinct 

 family of Batrachian reptiles, that I hesitate in absolutely pronouncing 

 it to be that of a fish. 



The teeth of the Saurichthys characterize the Bristol bone-bed 

 and the German Muschelkalk ; they are conical, rather slender, with 

 a subelliptical transverse section, slightly bent, with the apex not 

 very acute : about one fifth of the tooth next the apex is smooth and 

 polished, the remainder of the outer surface is traversed by fine close- 

 set longitudinal ridges : thus, to all outward appearance the tooth 

 very closely resembles the form most common among Saurians. 



The specimen here described, from the Bristol bone-bed, is seven 

 lines in length, and two broad at the base. I have investigated its 

 structure by a transverse section through its base, and by a longitu- 

 dinal section through the middle of the remaining part. 



This tooth is traversed by a slender, straight, longitudinal, 

 conical, central pulp-cavity, occupied by a coarse cellular bone, and 

 from which radiates a system of close-set, slightly and minute- 

 ly undulating calcigerous tubes, covered by a thick layer of the 

 enamel-like dense tissue, which coats the teeth in the Lepidotus, 



(1) Linnaean Transactions, vol. 18, 1839. p. 350. 



(2) The term Saurichthys (oavpog a lizard, ix^vs a fish) was applied by M. Agassiz to the 

 present genus, to express its transitional characters, but it is nevertheless regarded by that 

 distinguished Palaeontologist, as being essentially a member of the class of fishes. 



