THE BRACHIOPODA 35 



translucent shell or, better, on an internal cast), and by 

 the absence of the small foramen and deltidial plates of 

 the rhynchonellids. In this genus there is a dorsal as 

 well as a ventral median septum. 



The Pentameracea appear to spring from the Cambrian 

 Syntrophia. Other primitive genera (Clitambonites, Poram- 

 bonites) occur in the Ordovician of Russia ; the typical 

 pentamerids abound in the Silurian and Devonian; 

 Camarophoria endures to the Permian, after which the 

 whole sub-order is extinct. 



ORDER IV. : TELOTREMATA. 



Articulate Brachiopoda in which the delthyrium is re- 

 stricted by a pair of deltidal plates ; the brachia are 

 supported by calcareous crura, loops or spirals. 



SUB-ORDER i. Rhynchonellacea. Crura-bearing 

 Telotremata. Shell fibrous, not punctate. Deltidial 

 plates more or less embracing foramen, which is small, 

 often elliptical, and does not encroach on the umbo 

 (hypothyrid). Typical form short and stout (length, 

 breadth, and thickness not very unequal) with short 

 curved hinge-line, usually with a fold on the dorsal valve 

 and sinus on the ventral ; smooth or ornamented by fine 

 radial lines or strong radial ribs, sometimes by spines. 

 Teeth supported by dental plates. Cardinal process 

 usually wanting. 



The rhynchonellids can generally be recognized by 

 their form, though Tnplesia among strophomenids and 

 Camarophoria among pentamerids are similar. The 

 presence of a sinus in the ventral valve and fold in the 

 dorsal, with consequent V or W shaped plication of the 

 valve margin, is very generally characteristic of this sub- 

 order ; it is one method of separating the inflowing and 

 outflowing water-currents. 



In some cases they approach terebratulids in shape, 



