338 
PALAEONTOLOGY OF NEW YORK. 
The tendency to contract the pedicle-cavity and deltidium presents its 
extreme manifestation in the Devonian forms of Stropheodonta, Strophonella 
and Leptostrophia, where it has become almost, and sometimes quite obliter¬ 
ated, and the entire pedicle and umbonal cavity filled with testaceous secretions. 
Such filling can occur only in a discarded and useless space, after the pedicle 
has ceased to be functional. A morphological consideration of much importance 
presents itself here, as well as in many other groups of genera where the shells 
attain great size. The evidence is very direct from the study of the structural 
features as given above, that the entire muscular system on the ventral side of 
the body, is, in primitive forms, inserted upon the base of the pedicle-cavity. 
This is apparent from a study of such a shell as Orthis callactis, where it is per¬ 
fectly clear that no muscular bands were attached to the pedicle-valve outside 
the limits of this strong and condensed posterior area, which is but a sessile 
spondylium. The contraction of this pedicle-cavity is accompanied by (whether 
in relation of ciuse to effect can not be stated) a diffusion of the area of mus¬ 
cular attachment, and when the shells are large, as in Stropiiome.va, Rafines- 
QUixA, Stropheodonta, Ortiiothetes, Derbya, etc., the necessity for powerful 
muscles, or some similar cause, magnifies this expansion of the muscular area 
until the original contents of the pedicle-cavity may be represented by enorm¬ 
ous muscles whose scars extend almost to the anterior margin of the valve, as 
in IIipPARiONYX and Riiipidomella. 
In this great group of genera there are two types of contour, one, as in 
Lept^sna, being normally convexo-concave, that is, with the pedicle-valve con¬ 
vex and the brachial valve parallel to it and concave; the other, as in Stro- 
PHOMENA, having this contour reversed, the pedicle-valve at first convex, but 
subsequently and through all later growth-stages concave, while the brachial 
valve becomes correspondingly convex. In both cases, as in other brachiopods, 
the primitive and post-embryonic valves are both convex. The peculiar reversal 
of contour, which is never more extremely manifested than in this group, but 
nevertheless occurs in other genera, such as Atrypa, many Rhynchonellas, etc., 
is a purely secondary condition. Its causes have not been fully investigated, 
but an unequal peripheral growth of the two valves, arising from inequality in 
