109 



Mr. Walcott has borrowed from the writer the types of this species 

 and of Lingulepis Gregwa for study and has decided that the former is 

 the young of the latter.* While having the highest respect for Mr. Wai- Not the young- 

 cott's knowledge of Cambrian Brachiopoda, it may be well to present Gregwl? " 

 certain objections to this dictum. 



Students of the Brachiopoda will recognize that in Lingulella and allied 

 genera the young stages are often round, when the adults become elon- 

 gated by more rapid growth at the front than at the sides. In assuming 

 Leptobolus atavus to be the young of Lingulepis Gregwa Mr. Walcott 

 would reverse this law. Another objection to this decision is that L. 

 Gregwa does not occur in the Assises E. 3 d and E. 3 e, where L. atavus 

 is found but is much older ; and on the other hand L. atavus in the typical 

 form does not occur in the ( Assise E. 1 o?,) Gregwa shale. Other objections 

 might be presented, but these are sufficient. 



MUTATION TRITAVUS, n. mut. PI. VI, figs. 5 a-c. 

 In the Lower Etcheminian another form which may be referred to this ,. 



T HIT- Mutation 



species is found. It is smaller than L. atavus or L. insulce but proper- tritavus. 



tionately wider ; the edges of the valves have a radiating as well as con- 

 centric ridging. 



The following are the dimensions of some valves collected. 



DIMENSIONS of Leptobolus m. tritavus from Assise E. 1 d. 



Dimensions. 

 &c. of this 

 form. 



*Proced. U.S. Nat. Mus. vol. XXIII, page 694. 



