VERTEBRATES. 197 



, L. *, 



GEXUS PSAMMODUS, Agassiz. 



Psammodus, AGASSIZ, 1843, Poissons Fossiles, tome III, p. 110. 



Teeth quadrilateral or trapezoidal in general outline, variable, usu- 

 ally thick and massive. The coronal region presents a more or less 

 plane surface, according to the position the form occupied upon the 

 jaws, always arched, generally moderately, in the longitudinal direc- 

 tion or from behind forward, transversely concave (maxillary teeth), 

 or more or less convex (mandibular teeth), sometimes raised into a 

 low ridge along the exterior lateral border, also along the articular 

 inner border, or showing a more or less wide convexity in the latter 

 region, and sometimes presenting a more or less well defined trans- 

 verse prominence in mature maxillary form. The marginal limits 

 of the crown are well defined, rounded along the exterior of lateral 

 border and usually inbeveled, and almost always making an angu- 

 lation at the articular inner border and along the anterior and pos- 

 terior margins, the enamel extending well down and more or less 

 distinctly defined from the coarse vermicularly pitted basis, which 

 constitutes the greater part of the height of the tooth. In front and 

 behind, the basal wall is nearly exactly vertical to the plane of the 

 coronal surface and moderately channeled or concave; the inner 

 articular face is also vertical and slightly excavated, presenting gen- 

 erally at one or the other extremity an obliquely truncated articular 

 facet for co-adaptation with the contiguous tooth of the opposite 

 series, the extent and obliquity of the truncation varying greatly 

 according to the species ; the exterior lateral border, in typical forms, 

 shows an expansion of the basal portion beyond the coronal limits, 

 increasing in breadth and terminating in a more or less produced 

 spur at the postero-outer angle of the tooth. The coronal surface 

 exhibits under an ordinary lens a distinct vertical prismatic struc- 

 ture, each of the vertical columns enclosing a medullary tube the 

 appearance of which at the surface produces the exceedingly minute 

 punctation usually observed in these teeth ; the exceedingly elegant 

 vermiculose rugosity exhibited in the less worn surfaces of certain 



