THE ORDOVICIAN PERIOD. 357 



A comparison of the accompanying figures with those of the Cambrian 

 brachiopods will illustrate, in some degree, these evolutions. All these 

 devices seem to imply that the enemies of the brachiopods had in- 

 creased in effectiveness also, perhaps in somewhat like proportions. 

 That they did not gain undue mastery seems to be implied by the 

 abundance of the brachiopods. A curious form was the concavo- 

 convex type (Fig. 163, p and q), in which the living space between the 

 valves was reduced almost to a minimum. Perhaps this also was an 

 adroit protective device in that it gave so much shell for so little meat 

 that it was scarce worth while for their enemies to crush them. 



The abundance of the bryozoans. — The bryozoans were akin to 

 the brachiopods, but their external forms, their habits of life, and their 

 modes of depositing their hard parts were so different as to mask their 

 kinship in the fossil form. The bryozoans lived in colonies, connected 

 by a common mantle which secreted calcareous material to form the 

 base or skeleton of the colony. These secretions so closely resemble 

 those of the corals that they have often been classed as corals, and 

 differences of opinion still exist relative to certain forms, the Mon- 

 ticulipora for example. 1 Certain bryozoan colonies formed hemi- 

 spherical masses much like those of certain corals (Fig. 164, b) . Others 

 assumed branching forms, resembling the ramose corals (Fig. 164, 

 h and i), while still others formed networks spread over other fossils 

 (Fig. 164, c, d). The individual pit-like cavities, or cells (Fig. 164, 

 c, g, /, and j) occupied by the individual bryozoans are usually much 

 smaller than the cups of the corals, though the smallest cups of the 

 corals are not larger than the largest cells of the bryozoans. 



Judging by the fossils, the bryozoans were very rare in the Cam- 

 brian fauna, and were not very conspicuous in the early Ordovician, 

 but they became abundant in the middle and later portions of the 

 period, when they contributed an important element to the rock- 

 material. The branched forms predominated, but the hemispherical 

 and reticulated forms were not uncommon. 



The deployment of the echinoderms. — While there is little more 

 than a bare indication that the echinoderms were present in Cam- 

 brian time, before the close of the Ordovician period the cystoids had 

 mounted to their climax, the crinoids had become prominent, and 



1 "The Problem of the Monticuliporoidea, " F. W. Sardeson, Jour. Geol., Vol. IX, 

 1901 pp. 1-27. 



