404 GEOLOGY. 



stood the vicissitudes of the passage from the Ordovician to the Silu- 

 rian with no loss to the prestige of the class, though they suffered 

 an almost entire change of species and a very large change of genera . 

 When it is considered that the brachiopods were among the most 

 resistant and conservative of the invertebrates, this large change of 

 species and genera emphasizes the stress of the conditions that con- 

 trolled the transition, as well as the biologic importance of the interval 

 of transition. 



The brachiopods remained the most abundant organisms of the 

 Mid-Silurian fauna, and shared with the crinoids, corals, and cephalo- 

 pods the leading places. They had gained greatly in differentiation, 

 and had made some notable advances in structure. On the whole 

 they were more robust and gave more obvious signs of abundant vitality 

 than in preceding periods. The curious concavo-convex forms were 

 less abundant, though not wanting. There was a great increase in the 

 spire-bearing genera. One of these, the Spirifer, named from its 

 fine brachial spires, is especially interesting. It had acquired a double 

 provision against the forcing of its shell by developing a long hinge 

 line at the back and a strong interlocking fold and sinus at the front, 

 and, in addition, its biconvex shell was usually, though not always, 

 well plicated (Fig. 187, p). The hinge line was not very greatly extended 

 during the Silurian period, but became remarkably elongate in the 

 Devonian. The Spirifers appeared first in this period, but had their 

 greatest development in the two periods following. 



It is interesting to note that along with the progressive develop- 

 ments there were retrogressive modifications. Most of the brachio- 

 pods were mounted upon or attached by a pedicle which, while fixing 

 them to the bottom or to some object, gave them some degree of free- 

 dom. But the Crania lost the functional use of the pedicel at an early 

 stage in the individual's life, and became cemented to other shells, or 

 some like object, thus losing their freedom of motion. Notwithstand- 

 ing this, the family was more abundant than in preceding periods, 

 and the degenerate development was apparently advantageous. 



Among the beaked shells, the Pentamerus family became the most 

 conspicuous and characteristic type (and gave name to certain beds). 

 The Pentamerus oblongus (Fig. 187, o) was the largest and one of the 

 most common of the Silurian brachiopods. The Pentamerus, like the 

 Trimerella (Fig. 187, r, s), developed remarkable processes on the interior 



