56 CESTRACIONTS. 



tooth. The unobUterated area of the medullary canals was occupied 

 by a vascular pulp in the living animal, and the silicious matter with 

 which they are filled in the fossil, has received a dark stain, probably 

 from the colouring matter of the vascular pulp ; but the finer tubes, 

 from the want of this difference of colour, are in many parts obs- 

 curely visible, if at all. They are discernible in some situations 

 crossing the concentric lamellae at right angles to the central canal. 

 The chief difference between the appearance presented by the Ha- 

 versian canals of the tooth of Acrodus, and those in bone, is in the 

 absence of the radiated cells or corpuscles. At the base of the 

 tooth, there are cells interspersed with the medullary canals, irre- 

 gular in size and form, very minute, and appearing like simple 

 granules without radiating lines. The character of the main or 

 coarser canals and calcigerous tubes of the ivory of the tooth of 

 Acrodus, reposes on their undulating course, their rapid diminution 

 and branching, and the moderately acute angles at which the branches 

 are given off, except at the circumference of the tooth, where they run 

 nearly parallel to each other. The line of dem.arcation between the 

 coarser and finer ivory, is formed by a series of small cells having the 

 form and arrangement represented in PI. 16, fig. 3, in which many of the 

 finer branches of the coarse ivory terminate, and from which the 

 minute tubes of the enamel-like ivory commence. The superficies 

 of the tooth is slightly punctated ; the depressions, however, do not 

 correspond with the mouths of tubes, but with the interspaces of 

 whole groups of the coarser tubes. 



20. Hybodus. — The remains of the extinct genus of Plagiostomes to 

 which M. Agassiz has given the name of Hybodus occur in the 

 secondary formations from the upper red sandstone to the chalk in- 

 clusive. They consist of teeth and large osseous spines, which have 

 been discovered so associated together as to leave httle doubt that 

 the recent animal was armed with a pair of spines, one to each dorsal 

 fin, as in the Cestracion, and that the jaws were beset with closely 

 packed vertical rows of teeth, containing from six to seven in each 

 row, (PI. 11, fig. 1). The teeth are in the form of transversely elon- 

 gated depressed cones, and consist of two pretty equal parts, viz : a 



