28 PALAEONTOLOGY 



impressions and teeth are seen. The raised rims of the 

 divaricator-areas join on to the teeth and delthyrial 

 margin like rudimentary dental plates, and there is a 

 slight median septum. In the dorsal valve, the cardinal 

 process is short and bifid and fits closely under the 

 chilidium. Dental sockets are conspicuous. 



Allied genera to Leptana are Rafinesquina (with fine 

 radial striations alternately larger and smaller, and with 

 ridged muscle-areas) and Stropheodonta (with fine denticu- 

 lations along the hinge-line, and inconspicuous chilidium). 

 Neither of these has the concentric corrugations of 

 Leptcena. All three are represented by common species 

 in the Ordovician and Silurian systems. Davidsonia of 

 the Devonian fixed itself by cementation on to other 

 shells, and shows internal spiral markings which are 

 interesting as evidence that spiral brachia existed in this 

 extinct family. 



Of genera with reversed concavity, Strophomena might 

 be described as a reversed Rafinesquina, and Strophonella 

 as a reversed Stropheodonta. 



The sub-family Orthotetina (Fig. 7, f-i) is characterized? 

 in general, by well-marked dental plates, which may 

 extend far forward in the pedicle-valve sometimes 

 parallel to one another (Meekella), or diverging (Schell- 

 wienella) or converging to form a median septum 

 (Orthotetes). In Derbya, however, the dental plates are 

 greatly reduced, the septum extending to the apex of 

 the umbo. These four genera are Carboniferous and 

 Permian. Externally all the genera of this sub-family 

 are radially ribbed. Certain species belonging to different 

 genera are so closely similar externally, that it is im- 

 possible to discriminate between them without examining 

 the interior, for which the grinding of sections may be 

 necessary : such species are said to be homceomorphs of 



