608 



PISCES. 



[PRI.— PSA. 



Fig. 1162.— Pristicladodus 

 springer!. 



Pristicladodus, McCoy, 1855, British Pal. 

 Rocks, p. 642. [Ety. from the two gen- 

 era Pristis and Cladodus.] Teeth re- 

 s e m ble Clado- 

 d u s ; median 

 and lateral 

 cusps strong ; 

 lateral edges 

 sharp and more 

 or less undu- 

 lated. Type P. 

 dentatus." 

 springeri, St. John 

 & Worthen, 

 1875, Geo. Sur. 111., vol. 6, p. 255, Wa- 

 verly or Kinderhook Gr. 

 Pristodus, Agaesiz. [Ety. pristi*, a saw ; 

 odom, a tooth.] This genus has been 

 only doubtfully identified in America. 

 (?)acuminatus, St. John & Worthen, 1875, 

 Geo. Sur. 111., vol. 6, p. 402, Waverly or 

 Kinderhook Gr. 

 Psammodus, Agassiz, 1843, Recherches sur 

 les Poissons Fossiles, t. 3, p. 112. [Ety. 

 psammos, sand ; odous, tooth.] Teeth 

 quadrilateral or trapezoidal in general 

 outline, variable, usually 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, trans- 

 versely 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 transverse 

 prominence in mature maxillary form ; 

 the marginal limits of the crown are 

 well denned, rounded along the ex- 

 terior of lateral border, and usually in- 

 beveled, and almost always making an 

 angulation at the articular inner 

 border and along the anterior and pos- 

 terior margins, the enamel extending 

 well down, and more or less distinctly 

 denned from the coarse, vermicularly 

 pitted base 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 artic- 

 ular face is also vertical and slightly 

 excavated, presenting generally at one 

 or the other extremity an obliquely 

 truncated articular facet for co-adapta- 

 tion with the contiguous tooth of the 

 opposite series, the extent and obliquity 

 of the truncation varying greatly ac- 

 cording to the species ; the exterior 

 lateral border, in typical forms, shows 

 an expansion of the basal portion be- 

 yond 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 dis- 

 tinct, vertical, prismatic structure, each 

 of the vertical columns inclosing a 

 medullary tube, the appearance of 

 which at the surface produces the ex- 

 ceedingly minute punctation usually 

 observed in these teeth ; the exceed- 

 ingly elegant vermiculose rugosity ex- 

 hibited in the less worn surfaces of 

 certain species is produced by the 

 wrinkling of the enamel or external 

 layer, and which apparently has no 

 other relation to the medullary tubes 

 than to rudely define them in irregular 

 and transverse or longitudinal rows, 



Fig. 1163.— Psammodus crassldens. a. Median 

 dental plates; b, transverse profile; c, longitu- 

 dinal profile. 



the punctse rarely confluent, and the 

 rugose appearance becoming obsolete 

 or more or less obscured over the more 

 exposed parts of the triturating surface ; 

 the impression also prevails that the 

 tendency to rugosity of the coronal 

 surface increases with age, since this 

 appearance, so far as observed, seems to 

 be most prevalent and conspicuous in 

 large individuals belonging to the series 

 which have received several accessions, 

 the innermost individuals of which 

 have suffered little from the abrading 

 effects of trituration while in use ; but 

 it is not an essential character, as some 

 species evidently always remained quite 

 smooth in their coronal areas ; the 



