APPENDIX B. (OWEN ON TEETH OF ICHTHYOLITES.) 
635 
B. 
An Account of the Microscopic Structure of certain Fossil Teeth from the environs of Riga, by 
which they are determined to belong to the genus Dendrodus of Sauroid Fishes. By 
Professor Owen. 
The teeth, of which the following is an account of the microscopic structure, were procured by Mr. 
Murchison from M. Pander, who collected them in the environs of Riga, and were submitted to me for 
examination with a view more especially to determine whether they appertained to the class of Reptiles 
or to that of Fishes. 
By some of the palaeontologists of Russia these fossil teeth had been referred to the Saurian order, and 
they bear, in fact, a close external resemblance to the teeth of crocodiles ; but the absence of any definite 
proof of the existence of air-breathing Vertebrata anterior to the deposition of the magnesian limestone or 
dolomitic conglomerate rendered the question of the possible existence of Saurian reptiles in the Old Red 
Sandstone of Livonia one of far too high importance to be left in any degree questionable, through an 
opinion of the nature of a fossil tooth founded solely upon an inspection of its outward form. 
Ihe insufficiency of this kind of examination had, in fact, been made very manifest by the results of a 
microscopic investigation of the structure of some fossil teeth, transmitted to me a short time previously 
from Scotland by a friend who suspected them to belong to Saurians, but which were proved by their 
internal structure to be referable to the class of Fishes, and to be indicative of a new genus in the Sauroid 
family for which I proposed the name of Dendrodus. Those teeth were obtained from the central or 
eornstone division of the Old Red Sandstone of Scat-crag, near Elgin. They are described, and their 
microscopic structure illustrated, in the first number of the Microscopic Journal 1 , and in my ‘ Odonto- 
graphy’ 2 . 
I recognised in the fossil teeth from Riga a close external resemblance with those of the Dendrodus 
from Scat-crag, especially the Dendr. biporcatus. They presented the same conical, slightly compressed, 
subincurved form, the same subcircular base and obtuse summit, the same additional resemblance to the 
teeth of crocodiles in the two opposite ridges extending from near the base to the summit of the tooth ; 
but the rest of the enamelled surface of the Russian teeth bore a greater resemblance to those of Saurians 
in having fine elevated longitudinal ridges, in addition to the linear impressions which were observable in 
the Dendr. biporcatus. 
Fortunately, however, the difference between the internal structure of the teeth of true Saurians and 
of the teeth of Sauroid fishes is well-marked and easily recognisable. In the Crocodiles, recent and fossil, 
as well as in the Enaliosauria, the dentine or principal substance forming the body of the tooth consists 
of an uniform system of dentinal or calcigerous tubes, radiating from a slender central pulp-cavity to the 
periphery of the tooth, without any intermixture of vascular canals. In the Dendrodus there is a wide 
pulp-cavity, subdivided into irregular longitudinal canals or sinuses, from which a series of vascular canals 
radiate to the periphery of the tooth, sending off branches at right angles throughout their entire course, 
whilst the dentinal tubuli are continued from the slightly expanded terminations of these branches, and 
from the peripheral terminations of the radiating trunks. This high organization, arising out of the ex- 
tensive distribution of the vascular system through the body of the tooth, does not exist in any Saurian 
reptile ; the dental structure bearing the closest analogy to it, in the class of Replilia, being that highly 
peculiar one which characterizes the teeth of the gigantic Labyrinthodont Batrachians. The difference, 
1 8vo, 1841, p. 4. 2 Part ii. p. 171, plates 62 A, 62 B. 
