10 FOSSIL BRACHIOPODA 



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 ex- 

 hibits 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 Eecent 

 species, as is especially well shown in the genera Orhiculoidea and 

 DiscinisecL. 



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 Ehynchonelloid 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 evidenced 

 by the development of Terebratulina. 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 hori- 

 zontal 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 suc- 

 cessive stages through which it passes in embryonic growth are 

 chronologically parallelled by different genera, and are likewise 

 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 opeinng 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 



