THE ORDOVICIAN PERIOD. 355 



notable, as the pelecypods are the lower and less differentiated type, 

 though the hinged double valve is a device of higher order than the 

 single valve. Representative forms are shown in Fig. 162. 



The abundant development of the brachiopods. — Shells of brachio- 

 pods are the most common fossils of the Ordovician strata. This 

 arose no doubt partly from their special suitability for preservation; 

 but chiefly, we may safely assume, from their numerical superiority 

 in the Ordovician fauna. The record, as it now stands, shows more 

 individuals per species than in any other class except perhaps the bryo- 

 zoans. This wide deployment is the more notable since the brachiopods 

 were among the most conservative of all the classes found in the fossil 

 state. The lower inarticulate forms of the order, which predominated 

 in the Cambrian, continued through the Ordovician (as indeed they 

 have done to the present time) , but the higher articulate forms greatly 

 outnumbered them. The large development of the articulate class 

 was attended by a progressive evolution of the mode of articulation 

 in which two lines were followed. In some the length of the hinge 

 was increased, apparently affording a better means of resisting the 

 attempts of their enemies to reach them by sliding or rotating the 

 valves past one another (see Fig. 163, a, b, e, f, g, etc.). In others the 

 front of the shell opposite the hinge was notched so that the valves 

 were interlocked when closed, and thus more effectively resisted any 

 sliding upon one another (Fig. 163, d and u). This device was usually 

 best developed in the shells of narrow and weak hinge-line, where it 

 was most needed (Fig. 163, t-u). Some shells combined both the 

 long hinge-line and the interlocking front (Fig. 163, c-d). The hinges 

 themselves were also improved by the use of different adaptations, 

 not easily described briefly, but illustrated measurably in Fig. 163, b, e, 

 g, and q. In addition to these devices for preventing the opening of 

 the shell, there was generally a thickening of the shell walls, and in 

 many cases a ribbing of the exterior which gave strength without need 

 less weight, and at the same time facilitated the crenulation and inter- 

 locking of the front edge (Fig. 163, d and u). Correlated with these 

 developments was also a deepening and strengthing of the muscular 

 impressions, which implies a stronger and better muscular system (Fig. 

 163, 6, e, g, and i). The introduction of the brachidium (Fig. 163, z), 

 a calcareous support for the fleshy arms, is another notable advance. 



