CAMBRO-SILURIAN MICRO-PAL^EONTOLOaY. 31 



surface, though not well preserved, presents no indications of having 

 been monticuliferous, but small " maculae " or aggregations of meso- 

 pores were probably present. Zooecia comparatively large, prismatic, 

 with thin walls and polygonal apertures ; twelve or thirteen of those 

 of the averas-e size in five mm. At intervals there are groups of 

 larger size, many of them with a diameter of half a millimetre or 

 more. These again usually enclose small and irregular clusters of 

 mesopores, which vary greatly in size, some being quite as large as an 

 average zooecium. Their number in a given space is usually about 

 equal to that of the zooecia and, in this specimen, at any rate, are always 

 distinguishable from them by being closed. 



Vertical sections show an interesting though not unique feature, a 

 similar peculiarity being characteristic of Gallopora and one which 

 has been noticed in certain species of M'onticuUpora, namely, that the 

 proximal end of the tubes is, apparently always, crossed by numerous 

 diaphragms whose development ceases suddenly, when the tube or cell 

 may be said to assume the character of a true zooecium. In the closely 

 tabulated condition it must (as it appears to me) have existed as a ''meso- 

 pore." The mesopoi-e stage may have been relinquished at almost any 

 height in the zoarium, yet it appears to have occurred simultaneously 

 in nearly all the tubes, the sections showing rather sharply marked 

 zones, the tubes in one being of nearly equal size and with few or no 

 diaphragms, and in the next very unequal and mostly full of dia- 

 phragms. These zones are distinguishable even in fractures of the 

 -zoarium, the walls in the first being nearly smooth, while in the latter 

 or closely tabulated region they are more or less wavy. In the speci- 

 men under consideration six alternating zones may be seen. Dia- 

 phragms and mesopores are abundant in the second, fourth and sixth, 

 and few in the first, third and fifth. From this it follows that they 

 represent respectively, what in my ''American Palaeozoic Bryozoa" is 

 termed the " mature" and '' immature" regions. This division of the 

 zoarium into distinguishable zones is probably the most characteristic 

 feature of the Trepostomata. 



The only tangential section prepared exhibits the characters of the 

 zoarium in its mature efondition. Here the mesopores are distinguished 

 from the zooecia by being smaller, more irregular and of a darker hue 

 than the zooecia. Acanthopores seem entirely absent. 



This species is more nearly related to the European D. petropolitana, 

 Pander, than is any other form known to me from American deposits. 

 They differ somewhat in the tabulation of the tubes. 



Big Island, Lake Winnipeg, Mr. T. C. Weston, after whom the species 

 is named. 



The genus Diplotrypa, as now understood, embraces at least three 



