﻿ulrich-bassler] revision of paleozoic bryozoa 45 



only three species, .1/. quadrata (Rominger), .1/. subquadrata 

 riricli, and .1/. crassimaralis Ulrich, were known. Since that 

 time another form, occurring like the others, in rocks of Richmond 

 age, has been discovered. Finally, a fifth species was brought to 

 light through the res-arches of Mr. Bassler on the bryozoa in the 

 Niagaran Rochester shale of New York. The last species differs 

 from the Richmond species in having acanthopores that may be dis- 

 tinguished in very shallow tangential sections of fully matured 

 zoaria. In all the other species certain acanthopore-like spines, 

 which often occupy the junction angles between the zocecial aper- 

 tures, leave no definite traces in the interior of the zoarium, so that, 

 if they are really of the nature of acanthopores, neither their pres- 

 ence nor their nature is indicated by tangential sections. 



The principal characteristic of the genus is the rhombic form of 

 the axial tubes in cross-sections. In vertical fractures the axial 

 region exhibits alternating smooth and uneven spaces. The smooth 

 spaces represent the fiat side of a row of tubes coinciding with the 

 plane of the fracture. In the uneven spaces the angles and two sides 

 of each of the tubes are exposed. In the latter case the plane of 

 the sides of the tubes, and consequently of the rows, forms an angle 

 of about 45 degrees with the plane of the fracture. This change 

 in directions results from a periodic and simultaneous development 

 of new tubes along the margins of those parts of the axial region in 

 which the rows have the same direction. The interpolated new tnhes 

 maintain the triangular shape necessitated by the form of the spaces 

 at their disposal until the next period of gemination causes another 

 turn and the interpolation of another set of young tubes. 



The development of new tubes ceases entirely with the entrance of 

 the older tubes into the peripheral zone. This is true also of prac- 

 tically all other ramose Trepostomata, and as a rule it is only after 

 the cessation of gemination that the increasing space at the disposal 

 of the zocecia, which naturally results from the peripheral growth in 

 this zone, admits of the development of mesopores and acanthopores. 

 In most cases, probably, the mesopores contained a specially modified 

 kind of zooid, as did also the acanthopores, but when, as in the case 

 of Rhombotrypa, the interzocecial spaces have no walls of their own, 

 it may well be doubted if they indicate a degree of dimorphism com- 

 parable with those cases in which true and fully developed mesopores 

 occur. It may be admitted at once that it is often very difficult to 

 decide whether these interspaces in a given case are true mesopores 

 or not, but under all circumstances it is a point deserving more 

 serious consideration than has yet been allotted to it. 



