BRYOZOA. 409 



vertical section of the zoarium cuts a few tubes through the 

 -liter but most of them at a greater or less distance from it. 

 < )uly when the section passes through the extremely small visceral 

 cavity, is the usual overlapping appearance of cystiphragrns 

 >hown. and the narrow visceral cavity seen to be crossed by 

 diaphragms. When the section does not pass through the vis- 

 ceral cavity, and it rarely does, the diaphragms seem curved or 

 horizontal and to extend clear across the tube. The appear- 

 ance presented by cutting through an infundibular cystiphragm 

 i- shown at the top of the third tube from the right, in figure 

 6a of plate XLV. 



The general features of the zoarium are decidedly like those 

 of several Lower Silurian species of the genus. The large size 

 of the cystiphragms. the consequent slenderness of the visceral 

 cavity, and the apparent absence of acanthopores, are marked 

 peculiarities of the species. 



Position and locality: Hamilton group, Thunder Bay. Mich. 



HOMOTRYPA Ulrich, 1882. 



Jour. Cin. Soc. Nat. Hist, Vol. V, p. 240.) 

 (For generic diagnosis see page 370.) 



Already ten (?) species belonging to this genus have been de- 

 scribed, yet this is scarcely one-half of the species known to me. 

 The Cincinnati group furnishes the largest proportion of them, 

 and. like Monticulipora, only one species is known to have ex- 

 isted at any subsequent period. The characters of the genus 

 are remarkably persistent and nearly always easily recognized. 



HOMOTRYPA ARBUSCULA Ulrich. 



PI. XXXVIH, fig. 3. 3 e. 



Zoarium loosely attached to foreign bodies by a broad basal 

 expansion, from which grow a number of stout dendroidal 

 stems with freely inosculating branches. Stems diminishing 

 from twelve mm. in diameter at the base to three or four at 

 the free terminations. Surface smooth, with scarcely noticeable 

 clu.-ters of larger cells. Cell apertures oblique, angular and 

 sliirhtly elongated, varying in diameter from the average of 

 0.18 or 0.20 mm., to 0.3 mm., the diameter of those in the 

 51 



