DEVELOPMENT OF THE BRACHIOPODA 241 



found to agree with the chronological history of the class, as 

 well as with the anatomical and shell characters, and there- 

 fore it is believed to be a natural and reliable sub-division. 



The first and simplest type of pedicle-opening is in shells 

 with a posterior gaping of the valves, through which the 

 pedicle protrudes in line with the axis. It is shared more or 

 less by both valves, although, generally, the greater portion 

 of the periphery is included by the pedicle valve. The 

 genus Lingula and related genera afford types of this form 

 of pedicle -opening. 



The second type is characterized by a pedicle wholly con- 

 fined to the lower valve, and emerging at right angles to the 

 plane of the valves. In primary forms it is not entirely 

 surrounded by shell growth, but occupies a sinus, slit, or 

 fissure. A further specialization carries it quite within the 

 periphery, and it finally becomes sub-central. A serial illus- 

 tration of this type is presented in the genera Schizocrania^ 

 Orbiculoidea, Discinisca, Schizotreta, and Acrothele. The 

 group probably terminates with forms like Crania and Plioli- 

 dops^ as shown by the development of the dorsal valve and 

 from internal characters. The development of the lower 

 valve, however, has not been observed as yet in either of 

 these genera. 



The third form somewhat resembles the second. During 

 the first nepionic stage of shell growth the pedicle is en- 

 closed by the ventral valve and the pro-deltidium. The per- 

 foration remains sub-marginal, and does not tend to become 

 centralized as in the preceding group. The initial pedicle - 

 opening may be maintained by further growth, forming a 

 deltidium; or it may be merged into the hinge opening by 

 resorption of the shell or by pedicle abrasion. Ortliisina^ 

 Leptcena^ Strophomena^ Chonetes, and Stropheodonta furnish 

 illustrations of the first condition, and the second is repre- 

 sented in the groups of OrtJiis. 



The fourth type in its incipient stage marks a return to 

 the simple conditions of the first, but in early neanic stages 

 the pedicle is confined to the ventral beak, and deltidial 



16 



