42 PALEONTOLOGY 



description very closely, e.g., S. bisukatus of the Carbon- 

 iferous Limestone, or 5. verneuili of the Devonian. The 

 genus in the strictest sense ranges from Devonian to 

 Permian, but there is. an allied form, Delthyris, small and 

 coarsely plicate in the Silurian, e.g., D. crispus of the 

 Wenlock Limestone. 



Other genera closely allied to Spirifer are Martinia 

 (Carb.-Perm., without dental plates and with tendency 

 to smoothness of surface), Syvingothyvis (Upper Dev.- 

 Lower Carb., with very large ventral cardinal area, 

 facing in a direction at right-angles to the plane 

 separating the valves, instead of parallel to it as usual, 

 and a peculiar "split-tube" or syrinx between the dental 

 plates), and Cyvtia (Fig. 7, /, Sil.-Dev., with high ventral 

 area, and narrow delthyrium with pseudodeltidium per- 

 forated centrally for the pedicle). Not quite so near are 

 Cyrtina (Sil.-Carb.) and Spiviferina (Carb.-Jur.) with 

 punctate shell and ventral median septum supporting 

 dental plates : they resemble in form the previous genera 

 which they respectively resemble in name. All these, 

 and various other genera, are usually united into one 

 family Spiriferida. 



In the family Atvypida we find the only Ordovician 

 spire-bearers, such as Zygospira, in which not only are the 

 spirals short and simple, but their apices are directed 

 towards one another, so that they must have had a 

 median exhalent current as in normal brachiopods. 

 Thus in every respect they are the most primitive spire- 

 bearers. The later (Sil.-Dev.) and more familiar Atrypa 

 (Fig. 7, m) has the spiral cones parallel with dorsalward 

 apices. 



Lastly, the family Athyrida includes the forms with 

 most complex brachial skeleton. The spirals point away 

 from one another as in Spirifer, but they start by a sharp 

 bend back from the crura, and the jugum is never a 



