DEVELOPMENT OF BILOBITES 401 



shell and the cessation of areal growth in the neanic period. 

 The form known as B. bilobus, var. Verneuilianus, Lind- 

 strdm, from Gotland, shows a tendency to develop in the 

 opposite direction, as the lobation becomes more and more 

 pronounced with growth, and the shell exceeds in size the 

 normal species. The decrease in the lobation of B. varicus 

 is a degeneration towards an embryonic character, while the 

 arrested areal development produces a condition of partial 

 isomorphism resembling one of the higher groups of Orthis^ 

 such as RhipidomeUa (B. Michelini L'Eveille). 



From what has been stated, it seems evident that the form 

 typified by B. bilobus from the Niagara was, at that time, 

 not a very plastic type, and capable of only slight degrees of 

 variation or departure from the normal form. Naturally, all 

 the modifications which occur containing a differentiation of 

 the essential idea of tlie genus appear in the early history 

 of the group, and are found previous to the Lower Helder- 

 berg form. The latter species while losing, in a manner, its 

 bilobus expression at maturity, degenerates into forms resem- 

 bling ancestral and other groups. 



The material for the basis of this paper was collected by 

 the writer from the lower members of the Shaly Limestone 

 of the Lower Helderberg Group, along the top of the main 

 escarpment of the Helderberg Mountains, between Clarks- 

 ville and the Indian Ladder, Albany county. New York. 

 Half-grown and fully developed specimens of Bilobites vari- 

 cus Conrad, sp., can still be picked up in considerable 

 numbers in the soil formed of the decomposed limestones. 

 The species, however, is not so abundant as formerly. Pro- 

 fessor James Hall is authority for the statement {Pal. N. Y.^ 

 vol. iii, p. 493) that forty thousand individuals were col- 

 lected between 1843 and 1853, and about four thousand in 

 the four following years. The young specimens have been 

 obtained only by carefully examining the decomposed sur- 

 faces of the limestones, and by treating with hydrochloric 

 acid slabs of rock in which the fossils are replaced by silica. 



26 



