DEVELOPMENT OF THE BRACHIOPODA 239 



attached to a foreign object under the hinge-line of a large 

 mature specimen of M. truncata, thus forcing the axis and 

 plane of the valves into parallelism with the object of sup- 

 port. In this way the pedicle emerged at right angles to the 

 axis. The growth of the shell and the increase in the size 

 of the pedicle caused the latter to encroach on the substance 

 of the lower beak, forming a dorsal perforation or pedicle- 

 notch, which in this example amounted to an arc of 180. 

 As the ventral valve was the upper and the dorsal the lower, 

 with the pedicle -opening through the latter, only the abnor- 

 mal position of the shell can account for this anomalous 

 discinoid condition. In the development of Orbiculoidea, a 

 true discinoid genus, it will be seen that during the early 

 stages it had a straight hinge and marginal beaks (Plate XI, 

 figures 5, 6, 7). Then, from its procumbent position and 

 peripheral growth, the pedicle became more and more en- 

 closed by the lower valve, until in Schizotreta (figure 11) and 

 Acrothele (figure 12), the opening finally became sub-central. 



The resemblance between this form of growth and habit 

 and Anornia is very suggestive. Morse and Jackson have 

 shown that from an early normal, bivalve, hinged shell the 

 right valve in its subsequent growth surrounds the byssus, 

 which occupies much the same position and performs a func- 

 tion similar to the pedicle of Discinisca and Orbiculoidea. 

 Peripheral growth also causes the initial shell to recede from 

 the margin. Another instance is thus furnished of a dis- 

 cinoid habit in an organism otherwise entirely different. It 

 is therefore evident that the discinoid form is purely due 

 to the mechanical conditions of growth. Hence the writer 

 believes that any bivalve shell with the plane parallel to the 

 object of support, and attached by a more or less flexible, 

 very short organ, as a byssus or a pedicle, without calcareous 

 cementation, assumes a discinoid mode of growth. 



The conditions of radial symmetry and ostrean growth 

 were briefly mentioned in a preceding section, and need only 

 be cited here as resulting from the cemented state of fixation, 

 as shown in species of Thecidium, Strophalovia, and Crania. 



