352 0. K Beecher — Development of the Brachiopoda. 



history of the class, as well as with the anatomical and shell 

 characters, and therefore it is believed to be a natural and 

 reliable subdivision. 



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 genera 

 Paterina and Lingula 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 sur- 

 rounded by shell growth, but occupies a sinus, slit, or fissure. 

 A further specialization carries it quite within the periphery, 

 and it finally becomes subcentral. A serial illustration of this 

 type is presented in the genera Schizocrania, Orbiculoidea, 

 Discinisca, Schizotreta and Acrothele. The group probably 

 terminates with forms like Crania and Pholidops, as shown by 

 the development of the brachial valve and from internal char- 

 acters. The development of the lower valve, however, has not 

 been observed as yet in either of these genera. 



The third form is an accelerated derivative of the second. 

 During the first nepionic stage of shell growth, the pedicle is 

 enclosed by the substance of the ventral valve. The perfora- 

 tion remains submarginal, and does not tend to become cen- 

 tralized as in the preceding group. The initial pedicle opening 

 may be maintained by further growth, forming a pseudo- 

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

 resorption of the shell or by pedicle abrasion. Orthisina, 

 Leptsena, Strophomena, Chonetes, and Stropheodonta furnish 

 illustrations of the first condition, and the second is represented 

 in Tropidoleptus and in the groups of Orthis. 



The fourth type in its incipient stage marks a return to the 

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

 pedicle is confined to the ventral beak, and deltidial plates are 

 developed in the majority of species. These plates at maturity 

 may entirely limit the pedicle opening below, so that the pedicle 

 emerges immediately under the beak, or encroaches upon the 

 substance of the beak itself. This type of opening is shown 

 by Zygospira, Spirifer, Khynchonella, Terebratulina, Magel- 

 liana, etc. 



The only divisions of the class which have had continued 

 existence are the Arthropomata and Lyopomata, proposed by 

 Owen in 1858.* Subsequently, various authors gave names to 



* Encycl. Brit., 8th eel., vol. xv, p. 301, 1858. 



