C. E. Beecher — Development of the Brachiopoda. 351 



fore 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, Strophalosia, and Crania. 



A long pedicle accompanies elongate shells with short hinges. 

 A short pedicle causes extended hinge growth when the plane 

 of the valves is ascending or vertical, but a discinoid form 

 results when the plane of the valves is horizontal. 



Types of pedicle openings. 



M. Deslongchamps is one of the few writers who have 

 given much consideration to the characters of the pedicle 

 opening. His studies, although mostly confined to the tere- 

 bratuloids and later spire bearing genera, conclusively show 

 the importance of this feature.* In a recent paper by 

 the writer, f attention was called to the persistence and 

 embryonic features of this portion of the shell. " It has 

 been shown by J. M. Clarke and the writer, that all species, 

 so far as examined, possessing a true deltidium in the adult 

 state, show that it was gradually developed in early stages 

 of growth, by concrescence along the lateral margins of 

 an open triangular area. Also, that all species furnished 

 with a pedicle- sheath have it fully developed in the earliest 

 growth-stages which have been observed for these species, and 

 the subsequent growth of the individual does not materially 

 alter its general characters, except that it is sometimes retro- 

 gressive, the parts becoming atrophied or functionally obsolete. 

 A feature of such importance, and so intimately connected 

 with the embryonal growth of the shell, must be given con- 

 siderable significance in discussing the various genera in which 

 it is present or absent." At that time, the development and 

 true interpretation of these different features of the pedicle 

 opening and the early stages of the shell had not been studied 

 sufficiently, and a more general application of the principles 

 involved could not then be made. The results of later studies 

 give prominence to these characters, and show that they 

 furnish a method for an ordinal grouping of the genera of 

 brachiopods. This is found to agree with the chronological 



* Note sur le developpement du deltidium chez les brachiopodes articules. 

 Bull. Soc. Geol. France. 2 e Ser. T. XIX, pp. 409-413, pi. IX, 1862. 

 f This Journal, vol. xl., p. 217, Sept. 1890. 



