SCOMBEROIDS. 129 



and calcigerous tubes pervades the whole substance of the tooth in 

 many, and probably in all, of the Scomberoid Fishes, and not only in 

 those of the existing epoch, but the same beautiful and complicated 

 structure may be discovered with equal distinctness in the fossil teeth 

 of species and genera which are now extinct. At Plate 54 is given 

 a view of the reticulate disposition of the medullary tubes, as seen 

 by a low power, in a transparent longitudinal section of the fossil 

 tooth of a large Scomberoid Fish, from the eocene formation called 

 the London clay, at the Isle of Sheppey. The tooth, which I for- 

 merly described under the name of Dictyodus, {I) in reference to the ar- 

 rangement of the medullary tubes, belongs to the extinct genus of 

 fishes called by M. Agassiz, Sphyranodus, from its affinity to the 

 existing genus, Sphyrcsna, the largest species of which it must have 

 equalled in size. 



The natural form of this tooth, which is from the lower jaw, is 

 conical and subcompressed, but relatively thicker than any tooth of the 

 Sphyrsena : its base is broad, but implanted as in that genus, in a deep 

 socket, to the bottom of which it is anchylosed. The main body of the 

 tooth consists of a coarse dentine, incased by a thin layer of fine and 

 hard enamel-like dentine. The coarse dentine is pervaded by an as- 

 semblage of medullary canals, directly continued from the large irre - 

 gular medullary sinuses and cells at the anchylosed base, and thence 

 proceeding in a general parallel course towards the apex, sparingly 

 dichotomizing, and gradually diminishing as they proceed : they are 

 separated by interspaces, which are generally equal to three or four 

 of their own diameters, and send off, throughout their course, short 

 transverse branches, which anastomose and intercept in the interspaces 

 of the longitudinal tubes, quadrangular, sub-elliptical, pentagonal, 

 or hexagonal spaces, generally elongated in the axis of the tooth, 

 but becoming shorter as they approach the periphery, especially at 

 the apex, where the structure of the tooth resembles an irregular lace- 

 work. In this fossil I have been able to detect the fine calcigerous 

 tubes only in the hard peripheral layer of dentine, into which they 

 pass directly from the nearest medullary canals, dividing and subdi- 

 viding at acute angles, but with a general direction vertical to the 



(1) Trans. Brit. Association, 1838, p. 112. 



K 



