'250 NEW YORK STATE MUSEUM 



maintained through this early epoch in the existence of the species. In the 

 shales of the Genesee division, the only time at which the species becomes 

 highly abundant, the normal extreme is common and the second extreme 

 also not infrequent. But so common and gradual are the variations in 

 outline between these extremes that it is hopeless to look here for a 

 permanent distinction. We except from this statement unusually large 

 shells with sinuous contour occurring in the Styliola limestone and occa- 

 sionally in the shales, to which we have hereinafter given the name P. 

 sinuosa. The original Avicula fragilis of Hall was described 

 from the Genesee shales. The type specimen has never been redrawn, 

 and the generic characters of the shell are not well shown in the original 

 figure ; but the form portrayed shows the elongate oval outline of the 

 normal extreme. We ought, indeed, in dealing with the examples from 

 this horizon, to restrict the original species name, fragilis, to shells with 

 strictly this outline, and designate the opposite extreme by some species 

 designation, but, with the presence of all intermediate phases, the attempt 

 at distinction would here prove quite futile and confusing. In the Naples 

 fauna, where the shells are not very common, this distinction is immediately 

 practicable. Here the two extremes are well expressed, and, so far as our 

 observation extends, there are none of the intermediate stages. The two 

 extreme types of outline, both originating in the Marcellus, are here perma- 

 nently fixed and at once distinguished. In the fauna of the Genesee shales 

 the normal and restricted P. fragilis is surrounded by its variants, mobile 

 and unstable. With the passage of time the unstable means disappear, 

 leaving only the fixed extremes. The terms P. fragilis and P. f r a g i 1 i s 

 van orbicularis, in their application to the expressions of this type as 

 they occur in the Naples fauna, have therefore a definite significance with 

 relation to the species and its history. The figures of Lunulicardium 

 f rag i 1 e given by Hall [op. cit. pi. 71] are for the most part representa- 

 tions of the rounder and variant outline. Thus, figures 1, 2, 5-7, all 

 approach the outline of var. orbicularis; only figures 9 and 1 1 repre- 



