240 STUDIES IN EVOLUTION 



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 dis- 

 cinoid 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 exam- 

 ined, possessing a true deltidium [= deltidial plate] 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 [= deltidium] 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 retrogressive, 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 considerable significance in discussing the 

 various genera in which it is present or absent." At that 

 time the development and true interpretation of these differ- 

 ent features of the pedicle-opening and the early stages of 

 the shell had not been studied sufficiently, and a more gen- 

 eral 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 



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

 Bull. Soc. Ge'ol. France (2), XIX, 409-413, pi. ix, 1862. 

 t Amer. Jour. Set. (3), XL, 217, September, 1890. 



