BRACHIOPODA. 919 
StrRopsEODONTA, STROPHONELLA and LeprosrropHta, where it has 
become almost, and sometimes quite obliterated, 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 con- 
sideration 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 
veutral side of the body is, in primitive forms, inserted upon the 
base of the pedicle-cavity. Thisis apparent from a study of such 
a shellas Orthis callactis, where it is perfectly 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 cause to effect may not 
be stated) a diffusion of the area of muscular attachment, and 
when the shells are large, as in SraopHommna, Rarinesquina, 
StropHEoponTa, OrruorHeres, Drrpya, 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 enormous muscles whose scars 
extend almost to the anterior margin of the valve, as in 
Hirpartonyx and RuipipomE.a. 
In this great group of genera there are two types of contour, 
one, as in Leprxna, being normally convexo-concave, that is, 
with the pedicle-valve convex and the brachial valve parallel to 
it and concave; the other, as in SrropHomrna, having this con- 
tour 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 Arrypa, many Rhynchonellas, etc., 
is a purely secondary condition. Its causes have not been fully 
investigated, but an unequal peripheral growth of the two valves 
seems to be a sufficient explanation of its existence. As either 
the presence or absence of this reversal is a normal secondary 
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