SILURIAN BRACHIOPODA 45 



of Rhynchonellas, and not only on account of its minuteness, but also from 

 the entire absence of plications on its surface and from the elementary 

 character of the cardinal area, we are inclined to regard it as the actual ele- 

 mental or initial shell. 



DEVELOPMENTAL VARIATIONS 



General Form and Outline. The adult variations from the normal, 

 noticed above, seem to be in most instances, and probably would prove to be 

 in all, preceded by well-defined embryonic series leading up to them. 

 This must be the case, as the character of these variations, i. e., 

 variation in the number of plications on the median portions of the 

 shell, is such that they cannot be assumed, after the attainment of the adult 

 condition, as is possible in certain other forms of variation. But it is not to 

 be assumed that the conformation of the embryo which eventually pro- 

 duces any of these results, manifests them in the earliest stages of the 

 growth of the shell, rather, that the shells, under whatsoever variations at 

 maturity, all have the same unspecialized starting point. Hence, the fact 

 that some of these variations have not shown a complete series of immature 

 stages must be due to the insufficiency of our material, rich as it has been. 



Limiting our considerations now to the normal form with two plications 

 in the ventral sinus, we notice that the initial shell is smooth, and obcordate 

 in outline, with beak erect, while the mature shell is strongly plicate, strongly 

 ovate and with the beak sharply incurved. The transition from one 

 extreme to the other is through stages of growth between the limits 

 .65X.54 mm. (initial) and 12x12 mm. (average adult). In growth-stages 

 below 7x5 mm. dimensions, the shell is very depressed-convex, the dorsal 

 valve up to about this point, retaining a low, broad, median depression, 

 accompanied by a similarly low and broad median elevation on the opposite 

 valve. It is not always possible to determine with accuracy how many 

 plications are carried by this embryonic fold and sinus, on account of its 

 not being well limited, but their eventual reversion, in the adult shell, into 



