American Palaeozoic Bryozoa. 



235 



thickened, the diaphragms more crowded, and the greater number 

 of the ordinary tubes have along one or both sides a series of 

 C} 7 stoid diaphragms; now there is also developed a limited number 

 of much smaller tubes, which differ, at least near their point of 

 origin, from the ordinary tubes in having more closelv arranged 

 diaphragms. In consequence, the}' have there the usual appearance 

 of interstitial tubes. This character they may retain throughout the 

 zone, but as they enter the next succeeding " immature" zone, their 

 character has changed to that of an ordinary tube. The spiniform 

 tubuli can not often be detected in a section of this kind. 



A tangential or rather transverse section ma}' present three different 

 phases, according as it may pass either through the "immature'' (1st), or 

 fully ''mature" (3d) stage; or (the 2d) if it cut the tubes just as they en- 

 ter into the last stage. In the first, the tubes have excessively thin walls, 

 are always apparently of one kind only, and thoroughly simple. In the 

 second the walls are still very thin, and the appearance is like that of 

 the preceding stage, excepting that we now observe quite a large num- 

 ber of smaller cells, wedged in among the ordinary tubes. In the third 

 stage (PI. X., fig. 5), the walls have become appreciably thickened, the 

 smaller tubes, noticed in the second stage, have all, excepting a few 

 among the cells occupying the monticules, changed their character, so 

 that they can no longer be distinguished from the ordinary cells. This 

 stage is further marked by the development of a large number of very 

 small spiniform tubuli. Of the different phases above described, a 

 single section may show only one, or, if large, all three. 



The normal mode of growth of J/, mammulata, is unquestionably 

 the same as in other massive or discoidal forms of the Jfonticulipor- 

 idce. The frondescent examples of the species have an entirely different 

 structure from such truly frondescent forms as Heterotrypa frondosa, 

 D'Orb. (not Nicholson), or Homotrypa dawsoni (M. (Heterotrypa) daw- 

 soni, Nicholson). In the latter, as well as in all the ramose species, 

 the frond or branch is divided into an axial and a peripheral region, 

 and the structure of the tubes in these two regions, as is shown on 

 page 125 of this Journal, is widely different. No such difference can 

 be shown to exist between the axial and peripheral portions of any 

 frondescent specimen of Jf. mammulata. What we do find is precisely 

 similar to the structure and mode of growth observed in the massive 

 or lobate examples of the species, viz.: the ''immature' and "mature" 

 zones (respectiveh' equivalent to the axial and peripheral regions of 

 the ramose and truly frondescent forms), are reproduced at successive 



