242 THE PALEONTOLOGY OF MINNESOTA. 



[Homotrya slmilis. 



HoMOTRYPA siMiLis Foord. 



PLATE XX, FIGS. 38-33. 



Homotrypa similis Fooed, 1883. Contri. Micro-Pal. Cambro-Sil. Rocks, Canada, p. 10. 



Zoarium of medium size, ramose, the branches subcylindrical or compressed, 

 4 to 10 mm. in diameter, dividing at unequal intervals, often irregularly, occasion- 

 ally even anastomosing. Surface without monticules, but usually exhibiting well 

 marked substellate spots, consisting of aggregations of large, thick- walled cells, in 

 many cases surrounding a minutely granulose central space. Zocecial apettures 

 more or less oblique, the degree depending upon age, appearing thin- walled and 

 angular when in a good state of preservation, but much smaller and ovate, and 

 seemingly with much thicker walls, when slightly worn ; about twelve in 3 mm. 

 Mesopores wanting, acanthopores small, inconspicuous superficially. When per- 

 fectly preserved the walls are minutely granulose. 



Internal characters : Tangential sections will present a variety of appearances 

 depending upon the age of the specimen sectioned, and the distance from the sur- 

 face. In the central or deeper parts of a section prepared from an old example 

 (plate XX, fig. 32), the zocecia have thin walls, each will have a well-defined cysti- 

 phragm, or, if too deep to show the end walls distinctly, will be crossed by three or 

 four straight and curved lines, representing both cystiphragms and diaphragms. 

 From this condition we pass gradually into stages in which the walls are thickened, 

 the cystiphragms filled up more or less completely and their ends drawn out 

 and around so as to enclose a comparativ^ly small ovate open space. At the same 

 time the walls assume a minutely granular character, while at many of the angles 

 of junction a larger dark spot (acanthopore) is to be detected. These stages are 

 illustrated in figures 32 and 30. The lower half of the latter represents an unusual 

 condition, in having the original wall undefined. It should be remembered that very 

 few sections will show more than the first stage, and that, on account of the brevity 

 of the peripheral region and the obliquity of the zocecial tubes, it is at all times 

 rather difficult to prepare really satisfactory tangential sections. 



Figures 29 and 31 illustrate vertical sections taken from fully matured examples, 

 the first from Canada, the second from Minnesota. These and other sections show 

 that the tubes bend outward very gradually ; that they are tabulated throughout, 

 with the diaphragms from one to three times their diameter apart in the axial region. 

 As they near the periphery the diaphragms become oblique and curved and then pass 

 over into series of cystiphragms and short, crowded diaphragms, with from twelve 



