510 FOSSIL BRACHIOPODA chap. 



accelerated growth, by which features belonging to later stages 

 become impressed on the early embryonic shell. The most 

 variable and specialised valve — the ventral or pedicle valve — 

 naturally exhibits the effect of this influence first and to the 

 greatest extent. The Palaeozoic adult forms of many species 

 represent various pre-adult stages of the Mesozoic, Tertiary, and 

 Recent species, as is especially well shown in the genera Orhi- 

 culoidea and Discinisca. 



In the Strophomenoid shells the protegulum in the dorsal 

 valve is usually normal, but in the ventral valve abbreviation 

 of the hinge and curvature of the hinge line are produced by 

 acceleration of the " Discinoid stage " in which a pedicle notch 

 is present. 



No marked variation has yet been noticed in the spire-bearing, 

 or Terebratuloid, or Rhynchonelloid genera. 



The form of the shell and the amount of difference in shape 

 and size of the valves seem to be largely due to the length of 

 the pedicle and its inclination to the axis of the body, as evi- 

 denced by the development of Terehratulina. A series showing 

 progressive dissimilarity of the two valves arising from these 

 causes can be traced from Lingula to Crania. The greater 

 alteration that takes place in the ventral valve appears to be 

 due to its position as lower and attached valve. If the pedicle 

 is short a transversely-expanded shell with long hinge line results 

 when the plane of the valves is vertical or ascending, but when 

 the latter is horizontal a Discinoid form is found. This mode 

 of attachment is often accompanied by a more or less plainly 

 developed radial symmetry. Shells with long pedicles, on the 

 other hand, are usually longer than wide. 



The character of the pedicle-opening is of great significance 

 from an evolutional and classificatory point of view, for the 

 successive stages through which it passes in embryonic growth 

 are chronologically paralleled by different genera, and are like- 

 wise accompanied by the successive acquisition of other important 

 anatomical characters, as has been shown by Beecher and others. 

 The first and simplest type of pedicle opening is in shells with a 

 posterior gaping of the valves, where the pedicle protrudes freely 

 between them in a line with the axis, and the opening is shared 

 by both valves, though generally to a greater extent by the ven- 

 tral valve. Paterina (= Obolus lahradoricus) and Lingula furnish 



