18 AMEBIC AN FOSSIL BRYOZOA, [bull. 173. 



gemmation. Two unilaminar sheets growing erect, back to back, form 

 a hilatninar or hifoliate expansion ov frond. The epithecje of the two 

 layers of zooecia thus brought together form a mesotheca (mesial or 

 medial laminae). The small pores seen in the mesotheca, or between 

 the walls of adjoining zooecia of certain species when thin sections of 

 well-preserved specimens are examined under the microscope, have 

 received the name median tithuli (Ulrich). 



When gemmation takes places on any side of the zooecium, massy 

 forms result, which may be hemispherical, globular, or discoidal in 

 shape. Gemmation in a particular direction will produce ram^ose or 

 dendroid forms. 



Among the Fenestellidte and related families a network is formed. 

 The openings in the network are fenestr^des; ])ars connecting the 

 branches are dissepinnents. The surface upon which the zocecia open 

 is the obverse., the other the 7'everse. 



Monticules and macule. — It is quite common, especially in the 

 Trepostomata, for some clusters of zooecia to outstrip the surround- 

 ing zooecia in growth and tower above them. The rounded elevations 

 thus formed are termed monticules. Sometimes the clusters consist of 

 mesopores instead of zooecia, surrounded by a zone of larger zooecia. 

 In the latter case the clusters of mesopores may not be elevated above 

 the general level, but may be even depressed below it. Such clusters 

 are called macidce. 



Zocecium: (cell). — The cavity with its bounding wall inhabited by 

 the animal. The wall is constructed of laminated tissue. In particu- 

 larly well-preserved specimens, thin sections reveal very small tubu- 

 lar passages penetrating the walls of adjoining zooecia. To these 

 Ulrich has applied the term cotnmunicatimi jpores. 



In some forms, especially among the Trepostomata, the zocecia have 

 the form of elongated tubes, which are crossed by partitions termed 

 diaphragms. The opening upon the surface of the zooecium or of the 

 vestibule among the Cryptostomata is the aperture. It is often closed 

 by a zomcial cover (closure or operculum). 



One side of the zooecial cavity in some of the Trepostomata is lined 

 with a series of superimposed vesicles, the cystiphragms (cystoid dia- 

 phragms); their purpose or use is unknown. 



Among the Cryptostomata plates frequently project from the walls 

 into the cavity; that upon the posterior wall of the zooecium is the 

 superior Jiemiseptum (Ulrich), that upon the anterior wall is the infe- 

 rior hemiseptum. The opening of the zooecium among the Cryptosto- 

 mata is the orifice; the tubular shaft which is left above the orifice as 

 the surface of the zoarium is thickened by strengthening, or protective 

 tissue in the vestibule. 



Interspace. — The part of the surface of the zoarium between the 

 apertures of the zocecia. 



