32 PALEONTOLOGY 



In the Permian Strophalosia also, the ventral valve is 

 cemented to foreign bodies. 



The family Orthida may be illustrated by Dalmanella 

 elegantula of the Wenlock Limestone, a small shell, nearly 

 circular in outline, little over a centimetre long and about 

 as broad (Fig. 7, d). The length of the hinge-line is less 

 than the greatest breadth. The ventral valve is very con- 

 vex, with a well-defined slightly concave cardinal area, a 

 triangular delthyrium without any deltidium, and teeth 

 supported by dental plates. The dorsal valve is very 

 slightly convex, with a very narrow area, and a very small, 

 bifid cardinal process occupying the centre of a small 

 delthyrium and at the end of a slight median septum ; the 

 inner side of each dental socket is produced into what at 

 first sight looks like a tooth, but as these do not fit into 

 sockets in the ventral valve they cannot be teeth but must 

 be crura like those of the rhynchonellids. The surface is 

 marked by rather fine radial ribs, which here and there 

 increase in number by bifurcation. 



The absence of deltidium and presence of crura form 

 two important differences from the Strophomenacea and 

 appear to justify raising the orthids to the rank of a sub- 

 order O rthacea. 



SUB -ORDER 2. Pentameracea. The uppermost 

 Ordovician and lowest Silurian beds in parts of England 

 and Wales are much alike, both consisting of calcareous 

 sandstones often crowded with fossils which are largely in 

 the form of internal casts. Many of these fossils are com- 

 mon to the two systems, or differ by characters too slight 

 to be detected in the field ; but the geological surveyor 



