34 PALAEONTOLOGY 



great development of the little septum seen in Leptana. 

 At its inner edge this is continuous with a pair of diverg- 

 ing plates which on being traced to the hinge can be 

 recognized as greatly-developed dental plates. We have 

 already seen that in Leptcena the rims of the muscle-areas 

 are continuous with the dental plates, and it is evident 

 that in Conchidium the development of these plates must 

 lift the muscles right off the inner surface of the shell 

 into the median chamber. Such a pair of muscle-bearing 

 dental plates is called a spondylium. 



In the dorsal valve instead of a median septum there is 

 a pair of septal plates a little on either side of the middle 

 line ; from these diverge at a low angle another pair of 

 crural plates the free edges of which are in contact with 

 those of the dental plates for some distance from the 

 hinge, thus bounding the central chamber. 



Although Conchidium knighti is commonly referred to 

 the genus Pentamerus, that should only contain smooth 

 species (e.g , P. oblongus of the Upper Valentian or 

 Llandovery stage). Other genera closely allied are 

 Stricklnndinia (e.g., S. lens, of the Lower Valentian or 

 Llandovery), with straight hinge-line and without the 

 greatly-curved umbones, and Gypidula (e.g., G. galeata, 

 Wenlock Limestone) in which there is a median elevation 

 (fold) on the ventral valve, and corresponding depression 

 (sinus) on the dorsal, an arrangement found in many 

 brachiopods, and serving to separate the lateral areas of 

 the mantle, where the inhalent current enters, from the 

 central exhalent area. Less close is Camarophoria 

 (Dev.-Perm.) with the closest external resemblance 

 to a rhynchonellid, from which it may readily be 

 distinguished by the internal plates (seen through the 



