Cumings — Development of some Paleozoic Bryozoa. 67 



III. Development of Unitrypa. 



The genus Unitrypa is characterized by the presence of 

 transverse bars or scalee, connecting adjacent carinse at inter- 

 vals corresponding approximately to the spacing of the zooecia 

 along the branches of the zoarium. Otherwise it conforms 

 closely in habit and structure to Fenestella. The development 

 of this genus reveals certain altogether remarkable features in 

 connection with the origin of the scales, or carinal bars. 



JVepiastic. Stage. 



The nepiastic colony of Unitrypa is an object of exquisite 

 beauty, consisting of a delicate translucent cup wherein is 

 lodged the pentamerously branching trunk or stalk of the 

 zoarium, with its slender arching carinas and diamond-shaped 

 incipient scales (figs. 63-68). The presence of this perfectly 

 formed cup is altogether unique, and its intimate relation to 

 the carinal superstructure is beyond question. 



The earliest stages (ana- and metanepiastic) of Unitrypa are 

 indistinguishable from corresponding stages of Fenestella. 

 Yery early, however, the margin of the basal plate begins to 

 curve upward into a shallow cup or saucer. At this stage the 

 carinas have not yet made their appearance, or are represented 

 merely by slight ridges down the outer edges of the incipient 

 branches (fig. 69). At a corresponding stage of Fenestella the 

 carinas are strongly marked (fig. 47). The margin of the basal 

 plate continues to grow upwai'd until a deep cup is produced 

 surrounding the branching stalk of the zoarium (figs. 63-68). 

 Near the termination of this stage each branch (a'-e!) becomes 

 united by a thin vertical plate (the carina) to the margin of the 

 cup. This plate or carina sends out lateral processes about mid- 

 way from the outer wall of the branch to the margin of the 

 cup, at a point where the edge of the carina is abruptly deflected 

 downward" toward the wall of the cup (figs. 64, 65 and 69). 

 The plane of these lateral processes is about parallel to a plane 

 tangent to the wall of the cup where the carina joins it. In 

 later stages these diamond-shaped processes are seen to coalesce 

 midway between adjacent caringe, so as to form transverse bars 

 or scales. The processes are therefore the rudiments of scales 

 (fig. 70). The margin of the cup is frequently scalloped in a 

 manner conforming to the curvature of the basal margins of 

 these incipient scales. 



The primary branches originate at about the third or fourth 

 tier of zooecia (fig. 69). The basal plate is indented opposite 

 the aperture of each zooecium of the initial circle. The nean- 

 astic and later stages of Unitrypa are quite similar to corre- 

 sponding stages of Fenestella. The zooecia are carried up the 



