THE BRACHIOPODA 33 



knows at once that he has Silurian and not Ordovician 

 rocks before him when he sees casts of rounded outline 

 cut almost in half by deep fissures. These fissures in 

 the cast correspond to large internal plates in the shell, 

 such as only occur in the pentamerids, a group of 

 brachiopods which suddenly migrated into the British 

 area at the opening of the Silurian period. 



One of the most familiar pentamerids is Conchidium 

 [Pentamenis] knighti (Fig. 7, j) of the Aymestry Limestone. 

 This is a large shell, attaining a length of 7*5 c.m., and 

 then about 4 c.m. broad and 5-5 c.m. thick. Both valves 

 are highly convex, with the umbo of each greatly curved 

 over ; there is no cardinal area ; the delthyrium is broad 

 and bears a concave deltidium (often wanting) ; the hinge- 

 line is short and curved. The surface is marked by coarse 

 rounded ribs which increase in number sometimes by 

 bifurcation, sometimes by intercalation of a new rib. 

 There are no regular concentric markings, but occasional 

 stoppages in growth are indicated by growth-lines. 



It is not easy to extract a perfect specimen, because 

 the fossil splits so readily along the large internal plates ; 

 and it is not difficult to get an almost median section. By 

 combining what we see in such a section with the evidence 

 of a transverse section (Fig. 7, j', j") we can realize 

 that the mantle-cavity in the umbonal region and for 

 some distance forwards was divided into three chambers, 

 the middle one containing the body proper, the lateral 

 chambers probably containing the spiral brachia. To- 

 wards the anterior end the three chambers united. In 

 the ventral valve there is first a large median plate, a 



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