21 



The form is very close to Hallopora onealli danforthensis , Parks, of the 

 Danforth member of the Dundas formation of the Don River, valley. The 

 present form differs only in a somewhat less robust zoarium and in the manner 

 in which the tubes turn out toward the surface, as seen in longitudinal sections; 

 in H. onealli danforthensis they turn rather sharply outwards at the boundary 

 between the mature and the immature zones, while in H. onealli creditensis they 

 bend very gradually outward, the boundary between the two zones being scarcely 

 distinguishable. It should be stated, however, that certain specimens have 

 been found on the Credit which cannot be distinguished from the Don Valley 

 forms. 



Locality. — Credit river, Streetsville. 



Nos. 12152, 12153, Royal Ontario Museum of Paleontology. 



Heterotrypa definita, sp. nov. 

 Plate II, Figures 5 and 6; Plate VI, Figure 11 



The coarse branching zoaria of this well-defined species are found in great 

 numbers in the bryozoan reef in the Strophomena varsensis zone of the Erindale 

 member. 



The branches are large, commonly reaching 15 mm. in diameter. Maculae, 

 each one made up of about 20 mesopores, are arranged evenly over the surface 

 with an average distance of two millimetres between them. 



In tangential sections, H. definita resembles H. simplex maculosa of the 

 Streetsville member in that the acanthopores are arranged very regularly, one 

 to each angle of junction of the zooecial walls. It differs, however, from that 

 species in the distribution of mesopores. In H. definita they are gathered into 

 maculae as well as being distributed evenly throughout the whole zoarium, 

 while in H. simplex maculosa they are found only in the maculae. In addition, 

 the groups of mesopores are never surrounded by zooecia of larger than average 

 size in the former as they are in the latter. 



The most striking characteristic of this species is observed in the 

 longitudinal sections. At definite periods of the growth of the zooecia the walls 

 become so swollen that a long continuous line of swellings can be followed for 

 considerable distances along the same level through the zoarium, sometimes for 

 the whole length of the section, in one instance, a_ distance of three millimetres. 

 The diaphragms are comparatively numerous, an average of 10 being found in 

 each tube, but they are entirely wanting in the axial zone. In the same manner 

 as the swellings referred to above, the diaphragms occur at similar levels in the 

 various zooecia, giving the appearance of continuous horizontal lines in vertical 

 section. In some cases the lines of diaphragms coincide with the lines of swell- 

 ings, and in other cases they alternate with them. 



This species has been placed under the genus Heterotrypa on account of the 

 numerous diaphragms, thick walls, and general resemblance to undoubted 

 members of that genus from the Credit River section. In one feature, however, 

 it greatly resembles Stigmatella, namely, in the possession of the periodic thicken- 

 ings which is considered by Ulrich and Bassler as one of the characteristics of 

 the latter genus. 



The coarse, branching growth, the surface covered with maculae, and the 

 periodic thickening of the walls' form a combination of features by which this 

 form is easily distinguished from any other species of Bryozoa. 



Locality. — Mullet creek, Erindale. 



Nos. 12154, 12155, Royal Ontario Museum of Paleontology. 



