146 The Life-History of Spirife?- Icevis. 



However, the normal type of *S'. Jimbriatus, (see pi. XIV), of 

 the Hamilton and earlier periods, is distinctly plicated, and the 

 series of radiating lines are coarse, and the lines themselves strong 

 and wider apart, and the species does not average half the size of 

 a typical S. IcBvis. There is another fact suggestive further of 

 the relationship : the variations noticed in the individuals of S. 

 Jimbriatus include an obscuring or obliteration of all those special 

 characters by which the typical forms of the two species differ 

 conspicuously from each other. 



The form and proportions {a) vary in fimbriates to those 

 characteristic of IcBvis, — tj'picai fimbriat us being much broader 

 than typical Icevis. 



(b) The size is decidedly greater in S. Icevis ; but the Corni- 

 ferous representative of fimbriatus is larger than the Hamilton 

 representative ; and for the great size of Icevis, we must look to 

 some cause not yet known. 



(c) The beak is smaller and less over-arching in fimbriatus ; 

 but in this species the beak is variable, and as the shape ap- 

 proaches that of Imvis, — (/. e., in the shorter specimens) the 

 relative size of the beak is greater. In Icevis, also, there are 

 varieties found in which the beak is as^^mall and low, propor- 

 tionately, as in some specimens oi fimbriatus. 



The feature {cl) is characteristic of fimbriatus as well as of 

 leevis. 



(e) The aperture is triangular and very similar in both ; but 

 it is rare to find the pseudo-deltidium preserved in fimbriatus ; 

 still, what traces there are of it lead us to presume that it was 

 obtusely arching, as in tee vis. 



(/) The smoothness of surface in Icevis is perhaps its most 

 prominent specific character ; but the presence of obscure pli- 

 cations on the margin in large specimens in the lowest stratum, 

 is strongly suggestive, and leads us to suspect that the ancestors 

 of this form were plicated. If we study a complete series of 

 Spii^ifer fimbriatus, C, we find the species, in its typical form, 

 characterized by a few (the number varying) broadly rounding 

 (the prominences varying), radiating plications ; and a common 

 variation is the disaj)pearance of the plication from the beak and 

 swollen part of the shell, extending far down toward the margin; 

 and occasionally an individual may be found of full size, but 



