1 78 AN INTRODUCTION TO THE STUDY OF FOSSILS 



Membranipora (Fig. 70). Jurassic to present. 



This genus differs from Bugula in the arrangement of the 

 individuals in the colony. The colony has the form of a flat 

 expansion incrusting seaweed ; thus the zooecia, the individuals, 

 are arranged in one plane, irregularly, or in rows. Each indi- 

 vidual, as in Bugula, consists of a chitinous bag, but much shorter 

 and with the rim around its aperture calcified. These individual 

 zooecia are united to one another by their thickened walls to 

 form a flat, net-like framework. 



Sketch three zooecia of a dried specimen. 

 How many individual animals do these three represent ? 

 How do the zooecia differ from those of Bugula ? 

 Of w^hat is the skeleton composed ? 

 Where at present may living Membranipora be found ? 

 6. What conditions must exist for such a living form to be 

 preserved as a fossil ? 



Monticulipora (Fig. 71). Ordovician to Silurian. 



Colonial, occurring as a flat expansion of upright, parallel 



tubes of calcium carbonate. These tubes have thick, non- 



FiG. 71. — Monticulipora arborea Ulrich, from the Middle Ordovician of Minnesota. 

 A, portion of a colony, natural size. B, view of surface ( X g), showing the cups 

 (corallites, cor.) occupied by the soft bodies of the individual animals during life. 

 C, longitudinal section from the periphery of branch inwards through fifteen 

 corallites ( X i8) ; cor., corallites; tab., tabulae or diaphragms. (From Ulrich.) 



