108 



The variation in form shown above is largely the result of distortion in 

 the shells after they were imbedded. The following table shows the 

 form and other features of valves from the assise below. 



DIMENSIONS of Leptobolus atavus from Assise E. 3 d. 



Compared 

 with other 

 species. 



In this assise the valves are larger, and proportionately longer than in 

 Assise E. 3 e. 



This species is of about the same size and geological age as Lingulella 

 Jerruginea, Salt.; but if Davidson's figures are correct, that species has a 

 hinge area twice as long as this. It may be further observed that David- 

 son has included in his species forms from much higher zones of the Cam- 

 brian (Dolgelly Group, &c), but the characters are so vague, that any small 

 oval species may be referred to L. ferruginea. We are subject to the 

 dilemma of chosing between two, or several species which by form and size 

 represent the species named, hence if one is L. ferruginea the others are 

 not. 



Lingulella Granvillensis, Walcott, of the New York Cambrian, is of 

 about the same size as this, and approaches it in form, but the moulding 

 of the interior of the dorsal valve differs. 



