104 SYNOPSIS OF AMERICAN FOSSIL BRACHIOrODA. [nni.i,.87. 



The prodeltidium, or third embryoni(5 shell plate, is known in the 

 Atrematii, Neotremata, and Protreniata. I n the Atremata this becomes 

 attached to the dorsal valve, while in the Telotremata it is a])parcntly 

 not developed at all. In the Protremata it becomes attached to the 

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

 fies the pedicle oj)ening. For this and other ontogentic and morphologic 

 characters, Owen's terms Lyopomata and Arthropomata are abandoned. 

 The Atremata and Telotremata are provisionally arranged under the 

 snperordinal term Homocaulia, and the Neotremata and' Protremata 

 nnder IdioeanUa. 



Morphologic equivalents, or similar structural features, are devel- 

 oped independently, as follows: A spondylium in Obolacea, Lingulacea, 

 Pentameracea, and rarely in Spiriferacea ; 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 Craniacea, Strophomenacea, 

 and Si)iriferacea. 



