SILURIAN BRACHIOPODA 81 



berlandice. Other forms, notably those with elevated areas, such as .S. macro- 

 notus, S. medialis, together with Cyrtina and Cyrtia, present considerable 

 differences in the completed deltidium, due principally, it is believed, to the 

 internal thickening of the beak and the growth of the transverse septum. 



The deltidium of Spirifcr thus appears to be the exact homologue of 

 the same part in Tercbrat^da, Rhynchonella, etc., and to be radically differ- 

 ent from the so-called " pseudo-deltidium " of Strophomcna, Strophodonta, 

 Streptorhynchus, and allied genera, although this term has been currently 

 applied to both the Spiriferidae and the Strophomenidse. 



