EARLY PALEOZOIC BRYOZOA OP THE BALTIC PROVINCES. 47 



Among the Cryptostomata plates frequently project from the walls 

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

 superior Jiemiseptum, that upon the outer wall the inferior hemiseptum. 

 The opening of the zooecium among the Cryptostomata 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 is the 

 vestibule. 



Maculse. — ^A common surface character among a large number of 

 Paleozoic bryozoans and many more recent species is the occurrence 

 at regular intervals of areas made up of larger zooecia and more 

 numerous mesopores, or of mesopores alone. These areas or spots 

 of special development are designated under the general name of 

 Tnaculse. When the maculse are elevated, they are known as monti- 

 cules or tubercles, according to the sharpness of elevation. 



Mesopores (interstitial cells) . — Tubular structure occurring between 

 the zooecia; they are usually smaller than the zooecia and are angular 

 or irregular in outline in cross sections. The number and size of the 

 mesopores varies greatly sometimes even in the same species. If 

 diaphragms are present they are invariably more abundant in the 

 mesopores, thus affording a ready means of discriminating between 

 the zooecia and mesopores. The development of mesopores is prac- 

 tically restricted to the peripheral portion of the zoarium and their 

 use simply to fill up whatever spaces were left between the zooecia. 

 Mesopores are, therefore, simply cells filling the interzooecial spaces. 



An interesting development of mesoporelike cells has been noted 

 in many of the Russian massive bryozoans, such as Diplotrypa bicornis, 

 EstJioniopora communis, and Dianulites petropolitana. In old exam- 

 ples of these species the basal portion of the zoarium shows no true 

 zooecia, but in their place are numerous thick-walled, closely tabulated, 

 mesoporelike cells. With age more and more tissue is deposited 

 upon the walls until finally the openings of the mesopores are prac- 

 tically obliterated. Possibly the zoarium of such massive species 

 rested in muddy deposits during life, and normal zooecia developed 

 only where a food supply was available. The Russian specimens of 

 Leptotrypa Jiexagonalis which have been invariably found incrusting 

 a species of HyolitTies likewise develop a basal layer of such thick- 

 walled mesopores. In every case this layer is found only on the flat 

 side of the HyolitTies. Other species which incrust HyolitTies, such as 

 Mesotrypa expressa, likewise show this layer only on the flat side of 

 the shell. If this layer of smaller cells is developed only on the basal 

 portion of the zoarium, its occurrence on the flat side of .the HyolitTies 

 is probably indicative that the bryozoan attached itself to only dead 

 shells which would naturally drop to the bottom flat side down. 



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

 apertures of the zooecia. 



