SILURIAN BRACHIOPODA 71 



Variations in Outline. Two very distinct groups of forms are evi- 

 dent in this species, in one of which, (a) normal, the outline of the mature 

 shell is obcordate or subpentagonal. When immature, the anterior margin 

 is evenly circular, but in all cases the proportion of length to width is essen- 

 tially the same. Probably five-eighths of the specimens found belong to 

 this group. 



In a second group, (<$) long form, the shell is elongate-spatulate, and 

 proportionately deeper than the normal, but, with a single exception, indi- 

 viduals have not been observed to exceed a length of 10 mm. and a width 

 of 7 mm. This variation is so persistent that it appears to be well founded 

 genetically, and not merely an occasional occurrence. Tracing backward 

 from the mature shell to the earlier stages of development, we find both 

 this and the normal form merging into each other, hence both have had a 

 similar starting point. The long form, however, reaches maturity of 

 development at a very early age, and never approaches the size or propor- 

 tions of the normal adult. A tendency to obesity is especially noticeable in 

 the group (<$), the majority of such individuals being below the normal full 

 growth. 



A single adult example shows traces of broad, rounded plications on 

 each side of the fold and sinus, a singular condition in a species uni- 

 formly non-plicate. 



Incipient Form (plate 7, figs. 4, \a, 10, io). Shell 2.5 mm. in 

 length by 1.75 mm. in width ; elongate, oval ; beak elevated, straight, acute. 

 Pedicle-aperture of the ventral valve very broad, triangular, extending to, 

 but not encroaching upon the apex. Dorsal beak full, rounded, but incon- 

 spicuous. Valves convex just below the beaks, becoming depressed toward 

 the anterior margin. The shell is proportionally much narrower than the 

 adult form. 



The starting point of our series is precisely the same form of shell as 

 that taken for the incipient stage in the species Meristella rectirostra. 



