96 



SYNOPSIS OF AMERICAN FOSSIL BRACHIOPODA. 



[ BULL. 87. 



Articulation was developed aloug two independeut lines, and therefore 

 the terms Lyopouiata and Arthropomata have no phylogenetic signifi- 

 cance. The presence or absence of articulating processes Avas at one 

 time considered a fixed line, on either side of whicli all brachiopods 

 could be arranged," but now articulation is known to be nearly functional 

 in several lyopomatous genera, as in Spondylobolus, Trimerella, Mono- 

 morella, Tomasina, Barroisella, of the Atremata, and in Trematobolus 

 of the Neotremata. Among the Arthropomata, articulation is hardly 

 functional in Kutorgina, Schizopholis, Eichwaldia, and Dictyonella. 

 However, it appears probable that two su])erorders exist, each having 

 two orders. Atremata and Telotremata are the more primitive groups, 

 and agree in the following fundamental characters: Prodeltidium 

 attached to the dorsal valve or absent; pedicle opening primarily 

 unmodified, and generally closed later by calcareous plates secreted 

 by the ventral mantle extensions; presence of a functional pedicle 

 throughout the life of the individual (excei)t in Thecospira, Thecocyr- 



Tig. 1.— Diagram giving the geological distribution of brachiopod orders. 



tella, and Bittnerula); general presence of the "obolella stage" in the 

 ontogeny of atremate and telotremate species, and the development of 

 complicated calcareous brachial sujiports in the derived order. The 

 Neotremata and Protremata agree in having the prodeltidium attached 

 to the ventral valve with comjilete nepionic modification of the pedicle 

 opening; delthyrium often closed by a single plate secreted by the 

 Ijedicle and never by mantle extensions; the pedicle is very often lost 

 before maturity is attained, along with the development of new anchor- 

 ing adaptations; absence of the "obolella stage" and complicated cal- 

 carious brachial supports. 



Owen's superorders Lyopomata and Arthropomata have no basis in 

 nature, and should be dropped. It is to be hoped that students will 

 determine the complete embryology of Lingula, Discinisca, Crania, 

 Rhynchonella, and Terebratulina, for until more of the ontogeny of some 

 species of these genera is known, no satisfactory relationship whicli the 

 orders bear to one another can be established. However, it appears 

 probable that Atremata and Telotremata have superordinal relationship 



