102 CONTRIBUTIONS TO CANADIAN PALAEONTOLOGY. 



from the base of one side to that of the other, is still clearly visible, 

 and there is a certain amount of regularity in their disposition. In 

 the central portion the wearing down of the Bummit has destroyed 

 their continuity, and on the sides the corrugations are so much abraded 

 as to be nearly obsolete. Near the anterior end the corrugations on 

 the sides are more irregular in their shape and disposition than at any 

 other part of the surface, and those which correspond to each other, on 

 the two opposite sides, are seldom, if ever, exactly alike. Thus, on the 

 right hand side of the crown, one of the corrugations close to the front 

 branches three or four times below the middle, and its longest branch 

 bends inward at nearly a right angle to the main stem and to the rest 

 of the corrugations, but this is not the case with the corresponding one 

 on the opposite side, and in no part of the surface do the corrugations 

 cross each other sufficiently often to form a complete, or even partially 

 complete, network. Under a lens also, the external orifices of the den- 

 tinal tubuli are plainly visible through the polished transparent ena- 

 mel, and where the latter is worn away, as on the summit and at the 

 anterior end, the orifices themselves are exposed and appear as close- 

 set punctures of irregular shajje. 



Dimensions of the only specimen collected : maximum length of the 

 crown, nine millimetres and three-quartei'S ; actual height of the crown, 

 as measured in the centre, six mm. ; breadth of the crown at ils base, 

 also as measured in the centre, six mm. 



Swan Eivej-, below Thunder Hill, J. W. Spencer, 1874: one sjieci- 

 men, which consists of the whole of the crown and a small portion of 

 the roots of one of the palatal teeth. From the Niobi'ara group, or 

 upper part of the series. 



In its general shape, especially as seen fi'om above, and in the pecu- 

 liar ornamentation of its crown, this tooth apfiears to differ from those 

 of any of the previously characterized species of Ptychodus from the 

 Cretaceous rocks of North America, but, until a larger series of speci- 

 mens shall have been obtained, its specific relations must remain 

 doubtful. 



Lamna Manitobensis. (N. Sp.) 



mate 26, figs. 6, 6a and 6b. 



Perhaps a variety of Lama macrorhiza, Cope. 

 Cfr. Lamna macrorhiza. Cope. 1875. Vert. Cret. Form. West (Rep. U.S. Geol 

 Surv. Terr., vol. II)., p. Ii97, pi. 42, figs. 9, 10. 

 A. S. Woodward. 1889. Cat. Foss. Fishes Brit. Mus., 

 p. 399. 



Teeth rather small ; crown or enamelled portion of each tooth con- 

 sisting of a central and nearly equilateral principal cusp or cone, with 



