338 PALEONTOLOGY 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 groat 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 callacUs, 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 diffusiou of the area of mus- 

 cular attachment, and when the shells are large, as in Stropiiome.va, Rafines- 

 QUiNA, Stropheodonta, Orthothetes, 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 Rhipidomella. 



In this great group of genera there are two types of contour, one, as in 

 LepTjEna, 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 



