THE BRYOZOA BASSLER. 36,°) 



resents the fossilized remains of animal matter which filled this 

 space during the life of the organism. Occasionally this narrow 

 intervening area is occupied by a light-colored tissue, and in this 

 case the outer boundaries of the wall of each zooecium can be seen. 

 In certain genera of both divisions the amalgamation or the dis- 

 tinct character of the walls is difficult to determine, especially when 

 mosopores are numerous, but if the zooecia are in actual contact 

 there is little trouble in deciding the position of the particular 

 form under study. Doubt has been cast upon the value of this 

 differentiation in recent years, but even if the two divisions should 

 not ultimately prove natural they are at least quite useful. 



ORDER 4. CRYPTOSTOMATA. 



In this order the zooecia are usually short and have their orifice 

 concealed (cryptos, hidden) at the bottom of a tubular shaft or 

 vestibule which is surrounded by a solid or vesicular calcareous 

 deposit. The primitive zooecium is short and quite regular in its 

 outline, being p} 7 riform to oblong, quadrate or hexagonal with the 

 aperture anterior. This same characteristic is shared by the Cheilo- 

 stomata also and it is probable that the Cryptostomata are nothing 

 more than Paleozoic Cheilostomata. The Cryptostomata differ, 

 however, from the t} 7 pical members of the Cheilostomata, first in 

 having neither ovicells nor avicularia, second in the much greater 

 deposit of calcareous material upon the front of the zooecia, third 

 in the frequent development of successive layers of polypides, one 

 directly over the other, thus forming a continuous tube, and fourth, 

 in that whenever a zoarium attains an uninterrupted width of 

 more than 8 millimeters it exhibits clusters of cells, differing more 

 or less either in size or elevation from the average zooecia. The 

 last two distinctions are suggestive of the Trepostomata, but the 

 Cryptostomata differ chiefly in that the immature region (primi- 

 tive cell) is usually much shorter and the passage to the mature 

 region more abrupt, and that hemisepta occur at the bottom of the 

 vestibule. 



Some of the Cryptostomata are ramose and have long thin-walled 

 prismatic tubes in the axial region, with or without diaphragms, 

 precisely as in the ramose Trepostomata and Cyclostomata. They 

 are distinguished from both these orders, however, by the presence 

 of the hemiseptum, the incomplete plate which extends downward 

 and forward from the posterior side of the base of the vestibule into 

 the primitive cell. Sometimes a second hemiseptum is found spring- 

 ing from the bottom of the cell, in which case the latter is known 

 as the inferior hemiseptum and the former as the superior one. The 

 purpose of the hemisepta is unknown, although it is possible that 

 they served as supports for a movable operculum. 



