BEYOZOA. 239 



Homotrypa subramosa.l 



later Homotrypella (?) ovata, illustrated on plate XVIII. The mesopores are on the 

 whole smaller and do not separate the zooecia so co mpletely, and sometimes the dif- 

 ference in these respects is greater than is shown in the illustrations. The difference 

 between vertical sections of the two forms is better marked, the cystiphragms being 

 more numerous and diaphragms wanting in the axial region in the present species. 

 Homotrypa exilis is another closely allied form, but differs in the oppo'site manner, 

 the' mesopores being fewer (see plate XIX). 



This is an important stage in the line of development discussed on page 216. 



Formation and locality .—HiAAAl^ third of the Trenton shales at St. Paul and Minneapolis, Minnesota. 



Homotrypa subramosa Ulrich. 



PLATE XIX, FIGS. 21-2^ 



Homotrypa subramosa Vi.nicii, 1886. Fourteenth Eep. Geol. Nat. Hist. Sur. Minn., p. 81. 

 Homotrypa insignisTJiiRicn, 1886. Fourteenth Eep. Geol. Nat. Hist. Sur. Minn., p. 82. 



Zoarium subrambse, frequently though irregularly divided ; branches compressed 

 or subcylindrical, their extremities often bulbous. Size of branches varying greatly, 

 the smallest 4 or 5 mm. in diameter, the largest 6 to 9 mm. thick, and as much as 

 25 mm. wide. Average specimens are about 6 mm. thick and between 8 and 12 mm. 

 wide, with the total hight of zoarium rarely exceeding 60 mm. Surface without 

 monticules, nor are the clusters of large cells very conspicuous. Zooecia with rather 

 thin walls and polygonal, direct apertures ; twelve to fourteen in 3 mm. ZocBcial 

 apertures shallow, exposing the cystiphragms when in a good state of preservation. 

 These structures leave but a small opening, and when the fossil has suffered a little 

 from attrition (a frequent occurrence in the beds holding the species most abun- 

 dantly) in which case the true walls are obscured or cut away, the appearance is very 

 deceptive, the apertures seeming to be very small and oblique, and much the greater 

 part of the surface occupied by wall-substance. Acanthopores varying in number 

 and size, sometimes as numerous as two to each zocecium. More commonly the 

 number is little more than half that extreme. In many cases they are large enough 

 to constitute a marked external feature. In others, however, apparently in an 

 equally good state of preservation, they are so small that it is difficult to detect 

 them even with the aid of a good lens. 



Internal characters: Vertical sections show that the tubes proceed in a gradually 

 increasing curve from the axial region outward to the peripheral region, in which 

 thev are approximately at right angles to the surface ; that in the axial region the 

 tubes are rather large, with wavy walls, and crossed by straight or oblique dia- 

 phragms, either in zones or occurring at intervals varying from one to three times 



