Palcoiitological Speculations. — Gratacap. 22(^ 
vantages of form or habit work for the preservation and con- 
tinuity of the differing- species. Strengthening of hinge struct- 
ure, increased soHdity of the shell, lengthened siphonal tubes,, 
stronger and more muscular feet, flattened shells to avoid im- 
pact of currents, roughened or spiculate surfaces for attach- 
ment to mud, rocks, sea-weed etc., attenuaton of form for 
insertion in crevices, development in byssus, gaping mouths, 
shoulders, elongation, curving backs, are features that might 
gain accentuation, and variously help their possessors to im- 
proved prospects in the life-race. And certainly many of these 
features are conspicuous in a review of the Devonian lamelli- 
branchs, but specific distinctions based on a slightly more ac- 
curate form, a more nasute outline, a triangular form, a length- 
ened or shortened anterior or posterior margin, a variation in 
surface sculpture, a higher umbonal prominence, proximity 
or recession of beaks, while all doubtless justly weighed as 
spr-^cific marks, apparently have no conceivable reference to any 
particular advantage they bestow. They may be indeed steps 
in an advantageous march of modifications. 
ScJiizodus shows a lengthening posterior e6.ge, and b;)th 
increase and decrease of the ridge, Cimtaria apparently be- 
comes more curved dorsally with an explanate scooped out 
widening of its posterior end, Mytilarca becomes more acute 
umbonally, elongate, and compressed, the pterinoid genera 
show both increase and decrease of the wings, accentuation 
and softening of the surface sculpture, Goniophora becomes 
generally from the Schoharie, more expanded, with a less 
arcuate umbonal keel. Ptcrinea Glabella found in the Upper 
Helderberg, but rare, is numerous in the Hamilton. It as- 
sumes a larger form with strong broad ribs in the Chemung, 
while around Aviciilopcctcn princeps are grouped oscillations 
of related forms. Pancnka and Liiniilicaniiuiii were hardy 
genera surviving changed habitats. 
The expansion of the Devonian lamellibranchiate fauna 
which seemed to begin in the Lower Helderberg was success- 
ively interrupted, and after each interruption widened its 
specific limits. Did the interruptions which expelled it to 
more restricted areas for development bring about the biolog- 
ical schock that caused the increase in species and a differen- 
tiation of new genera? It seems probable. 
