128 SCOMBEROIDS. 



typifies that of the teeth in all the Scomberoid fishes. It consists of 

 a thin external coating of fine enamel-like dentine, and a body of 

 coarse dentine. The most successfully injected vascular membrane 

 could not surpass in the number and delicacy of its ramified capillaries 

 the arrangement of the medullary canals, which pervade the whole 

 body of the tooth in this fish. The smallest marginal or peripheral 

 divisions of the medullary canals anastomose in curved loops whose 

 convexity is turned towards the superficies of the tooth ; the branches, 

 which, by their terminal division, constitute those loops, successively 

 anastomose, as they widen and slightl}'^ converge towards the central 

 part of the tooth. (1) Here the main medullary tubes run nearly 

 parallel to each other, with interspaces equalling from four to six 

 times their own diameter. In their progress towards the base of 

 the tooth, they give off numerous branches which subdivide and 

 anastomose in the interspaces of the main canals. These canals, in 

 the fully formed anchylosed teeth, communicate directly with the 

 medullary canals of the jaw-bones, which run at right angles with those 

 of the teeth. (2j The reticulate medullary canals every where send off 

 the much more minute calcigerous tubes, which quickly subdivide into 

 tufts or pencils ; these soon begin to bend and interlace together and 

 fill the meshes of the medullary reticulations by their inextricable anas- 

 tomoses. The peripheral loops of the medullary canals, in like manner, 

 give off pencils of fine calcigerous tubes ; but these maintain, as do the 

 tubes of ordinary dentine, a more regular and parallel course ; they 

 penetrate, and constitute with the clear uniting substance, the exter- 

 nal hard and white coating of the tooth ; the first two-thirds of their 

 course is at right angles to the plane of the surface towards which 

 they are directed ; their finer ramifications then begin to bend from 

 side to side, and terminate in reticular inosculations of extreme 

 minuteness, which are finally lost in the clear dense substance 

 of the outer layer. 



Sphyrcenodus. — The same general distribution of the medullary 



(1) A diminished view of the structure, as seen in a longitudinal section of the apex of 

 the tooth, is given at fig. 2, pi. 53. 



(2) Tlie solid tissue of the maxillary bones presents a close-set parallel arrangement of 

 undulating calcigerous tubes, and nearly resembles the texture of the dentine in the higher 

 mammiferous teeth. There are no Purkingian corpuscles or radiated cells in this bone. 



