VEBTEBBATES. 201 



with the associated forms. They plainly held a lateral position, and 

 from the character of the articular border, especially as displayed 

 in mature individuals, they apparently alternated with a row of me- 

 dian symmetrical teeth, of whose remains, however, not a vestige 

 has been detected in the collections accessible to us. In the event 

 of the above inference proving well founded, the latter form would, 

 with its associates, present a dental formula entirely unique and dis- 

 tinct from what obtains in typical Psammodus. There is marked 

 diversity amongst the latter, as will be made apparent in the sub- 

 joined descriptions of species. 



Attention is also directed to certain resemblances existing between 

 the Psammodi and Cochliodonts . As is well known, the teeth of 

 the latter were disposed on distinct rami of the jaws, instead of 

 spanning the jaws from side to side, as is the case with the 

 Psammodonts. But in respect to the outer lateral borders of certain 

 forms of teeth representative of the two families, characteristics are 

 observed which hardly are to be relegated to the category of mere 

 resemblances, and the presence of which suggests interesting homolo- 

 gies between the respective forms belonging to either group. In 

 this way the projection of the basal rim beyond the coronal border 

 in the external border and its prolongation into an acute spur 

 terminating at the posterior extremity, are unquestionably the same 

 in the exterior and posterior terminal forms of either family, the 

 posterior teeth of Cochliodonts homologicaiiy corresponding with the 

 form constituting the exterior rows of teeth in the Psammodonts, 

 even the direction of the obliquity of this side adding stress to the 

 likeness in which the forms of both families share. The succeeding 

 forms of the Cochliodonts find their representatives in the inner 

 rows (where such exist) of the Myliodonts, and in those genera of 

 the latter in which the jaws are arched by a single row of dental 

 plates. The condition is such as would be produced were the rami 

 of the Cochliodont jaws drawn out into a transverse position and 

 their dental armature consolidated into single plates stretching from 

 side to side across the symphysis. Indeed, in these early forms a 

 much more intimate relationship exists between the types of these 

 two great orders of Selachians than obtains between their represen- 

 tatives of later geological times. 



The genus is strictly of Carboniferous origin and duration, indeed 

 beginning nearly at the dawn of the earlier period and ceasing at 

 its close. The Coal Measures thus far have not yielded a trace of 

 the remains of Psammodus. 



