350 O. E. Beecher — Development of the Brachiojpoda. 



Professor Morse also called attention. The same author next 

 showed that the succeeding stage had a comparatively long 

 pedicle, and a shell linguloicl in form. Afterwards, the defin- 

 ing of the pedicle opening, shortening of the pedicle and 

 truncation of the ventral beak, produced the final characteristic 

 external features of Terebratulina. The deduction from this 

 example and from Lingula is, that genera having pedicles 

 sufficiently long to admit of freedom of axial movement have 

 elongate and rostrate shells. The shortening of the pedicle 

 brings the posterior part of the shell in more or less close 

 proximity to the object of support, and, as growth cannot take 

 place in that direction, it increases laterally, resulting in 

 broader forms with extended hinge areas, as in many species 

 of Cistella, Scenidium, Muhlfeldtia, Terebratella, Kraussina, 

 etc. 



The variety known as Muhlfsldtia ti'uncata, var. monstru- 

 osa, Davidson, further shows how discinoid characters may 

 be produced in an entirely different type of shell. A specimen 

 was found by the writer in a position which readily gave the 

 solution to its variation from the normal species. It was 

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

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

 plane of the valves into parallelism with the object of support. 

 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 abnormal 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 XVII, figures 5, 6, 7. Then, from its 

 procumbent position and peripheral growth, the pedicle be- 

 came more and more enclosed by the lower valve, until in 

 Schizotreta (fig. 11) and Acrothele (fig. 12), the opening finally 

 became subcentral. 



The resemblance between this form .of growth and habit 

 and Anomia 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 function 

 similar to the pedicle of Discinisca and Orbiculoidea. Periph- 

 eral growth also causes the initial shell to recede from the 

 margin. Another instance is thus furnished of a discinoid 

 habit in an organism otherwise entirely different. It is there- 



