550 



GEOLOCV 



abounding vitality than before; but along with the prorn^<ivc 

 developments there were some retrograde" modifications. 



The bryozoans. The coral-like bryozoans contributed much less 

 to the Silurian limestones than to those of the preceding period. 

 This was partly because the class had declined, and partly because 



Fig. 401. Silurian Brachiopods: a, Pentamerus oblongus March., lateral 

 view of an interior cost; 6, Trimeretta acuminata Bill., the interior 

 view of a pedicle or ventral valve, showing the elevated platform for 

 muscular attachment excavated beneath; c, Spirifer niagarcnsis (Con.), 

 exterior view of the brachial or dorsal valve, with the cardinal area 

 and beak of the pedicle valve showing above; <l, Chonetes corn nl UK 

 (Hall), exterior view of ventral valve, showing the cardinal spines; 

 e, Trimeretta ohioensis Meek, the internal cast of a highly differentiated 

 inarticulate brachiopod, showing the finger-like casts of the excavations 

 beneath the elevated muscular platforms; /, Stropheodonta profit n<l<t 

 Hall, interior of the ventral valve, showing muscular impressions; 

 g, Spirifer radiatus Sow.; h, Streptis grayi (Dav.). exterior view of the 

 brachial valve, showing the cardinal area and beak of the opposite 

 valve, and the peculiar twisted form of the shell. 



the more massive types were replaced largely by more delicate and 

 fragile forms. 



The moilusks. The cephalopods appear to have remained the 

 most powerful inhabitants of the seas. The straiu;l it forms were 

 still common, but curved and coiled ones were more numerous. 

 Their shells were more highly ornamented than before, but they 

 were still plain in comparison with some of their successors The 

 apertures of the shells of the Ordovician species were usually circu- 



