104 SYNOPSIS OF AMERICAN FOSSIL BRACHIOPODA. [bull. 87. 



The prodeltidium, or third embryonic shell plate, is known in the 

 Atremata, iS^eotreniata, and Protieniata. In the Atremata this becomes 

 attached to the dorsal valve, while in the Telotremata it is apparently 

 not develoi)ed at all. In the Protremata it becomes attached to the 

 ventral valve, as in Xeotremata. In the two last-named orders it modi- 

 ties the pedicle oi)ening. For this and other ontogentic and morphologic 

 characters, Owen's terms Lyopomata and Arthroi^omata are abandoned. 

 The Atremata and Telotremata are provisionally arranged under the 

 superordinal term Momocaidia, and the Neotremata and l*rotremata 

 under Idioeaulia. 



Morphologic equivalents, or similar structural features, are devel- 

 oped independently, as follows : A spondylium in (3bolacea, Liugulacea, 

 Pentameracea, and rarely in Si)iriferacea ; crural processes in Pentam- 

 eracea and Rhynchonellacea; functional articulation in Protremata 

 and Telotremata; straight, more or less long, cardinal areas from ros- 

 trate forms in Rhynchonellacea, Spiriferacea, and Terebratulacea; 

 rostrate shells from long cardinal areas in Pentameracea, and loss of 

 pedicle and ventral shell cementation in Crauiacca, Strophomenacea, 

 and Spiriferacea. 



