130 SCOMBEROIDS. 



surface of the tooth. The relative thickness of this layer of fine- 

 tubed dentine is shown in the figure ; the diameter of the calcigerous 

 tubes at the middle of the layer does not exceed isjooth of an inch. 

 The traces of the corresponding tubes in the clear interspaces of the 

 ramifications of the medullary canals were almost obliterated in the 

 fossil examined by me, but the analogy of the recent Scomberoid teeth, 

 and of some allied fossils, especially that about to be described, hardly 

 permits a doubt as to their existence. 



Saurocephalus. — There are few instances in which the value 

 of the characters derived from the microscopic structure of teeth 

 has been more strikingly displayed than in regard to the Sauro- 

 cephalus and Saurodon, under which names two interesting fossils of 

 distinct species, of the same or very nearly allied genera, have been 

 described as extinct members of the Saurian Order of Reptiles, by 

 Dr. Richard Harlan (1) and Dr. Isaac Hays. (2) 



These fossils consisted of portions of jaws with teeth of a simple 

 conical subcompressed form, arranged in a single row, fixed in dis- 

 tinct and deep alveoli, each with a broad and simple fang, generally 

 excavated by the pressure of a new tooth developed near its base. 

 Prof. Agassiz was led from the external characters of the jaws and 

 teeth in question to believe that they might belong to the Scombe- 

 roid family in the class of Fishes, and an inspection of Plate 55, 

 which gives a view of a small portion of a transparent longitudinal 

 section of one of the teeth of the Saurocephalus, will demonstrate the 

 accuracy of the judgment of that acute Palaeontologist. 



I am indebted to Dr. Richard Harlan for the opportunity of 

 making the requisite sections of the tooth of the genus in question. 

 The plan of structure closely corresponds with that already described 

 in Sphyrcena and Sphyranodus. The larger medullary tubes {a a, 

 PL 55), maintain a nearly parallel longitudinal course, throughout 

 the body of the tooth, exhibiting here and there a dichotomous sub- 

 division, and gradually decreasing in diameter as they approach the 

 summit of the tooth. The lateral or transverse branches are, upon 



(1) Journal of the Academy of Natural Sciences of Philadelphia, vol. 3, p. 331. 



(2) Transactions of the American Philosophical Society, vol.3, part 2, p. 471. In the 

 Saurian system of Herm. v. Meyer these genera are placed between Phytosaurns and 

 Teleosaurus, see his Paleeologica, 8vo. 1832, p.p. 114, 222. 



