THE SILURIAN PERIOD. 405 



of the shell for the apparent purpose of shortening the muscles and 

 giving them better attachment, and also to better adapt the interior 

 to the visceral parts. Similar structural devices appear in other families. 

 This mode of securing economy or advantage appeared in Ordovician 

 times, though it was then rare. 



Three brachiopod species which survived the vicissitudes of the 

 transition from the Ordovician have already been mentioned. In 

 the St. Lawrence embayment, where there was less disruption of 

 continuity, at least two other species passed from the earlier period 

 to the middle of the later. In Europe, the transition seems to have 

 been easier, and more species survived it. 



The decline of the bryozoans. — The coral-like bryozoans con- 

 tributed much less material to the Silurian limestones than to those 

 of the preceding period. This was partly because the whole class 

 had declined in abundance, and partly because the more massive coral- 

 line type had fallen off relatively, and was largely replaced by the 

 more delicate reticulate forms, such as Fenestella (Fig. 188, / and g). 

 This change was probably advantageous, since a given number of 

 individuals arranged in such a plexus would compass a larger area 

 of food supply than in the more crowded form of the massive type 

 and at the same time would not seriously sacrifice their connection 

 with one another. The incrusting forms of bryozoans became com- 

 mon, and often covered entirely the shells of mollusks and brachiopods. 



The continued dominance of the cephalopods. — Except as possibly 

 rivaled or overmastered by the little-known vertebrates, the cephalo- 

 pods remained the most powerful of the known inhabitants of the 

 Silurian seas. The large straight orthoceratites were less prevalent 

 than before, though still common, while the curved and coiled forms 

 were more numerous. The orthoceratites were more varied in external 

 markings, some remaining smooth, some being fluted longitudinally, 

 and some being annulated by rounded rings or wavy lines (Fig. 187, I), 

 while others, by a combination of longitudinal and transverse mark- 

 ings, developed a reticulate ornamentation. Curved forms were the 

 most numerous, and fully coiled forms were common. These were 

 ornamented externally like the straight forms, but they were still 

 quite plain in comparison with the remarkable ornamentation which 

 the coiled forms assumed in later times. The apertures of the cephalo- 

 pod shells of the Ordovician were usually circular or oval, but in the 



