GEOLOGY OF STEEPROCK LAKE. 19 



Pores. — Pores appear in the walls between the tubes, but none 

 have been seen in either the inner or outer wall. 



Exoihccal Groivth. — The presence of e.xotheeal tissue somewhat 

 similar to that so common in Arcluvocyaihus is suggested by some 

 specimens, but it is not sufficiently clear to warrant giving it as a 

 character of the genus or species. 



Growth. — The mode of growth appears to have been individual, 

 although, as illustrated by fig. 5, PI. I, two central cavities appear 

 in what would otherwise have been" considered an individual. The 

 radial tubes are more or less confused where those that radiate from 

 the two cavities come in contact. My present impression is that the 

 two grew side by side, with only a very slight distance between 

 them, when small; as they grew, the central cavities were crowded 

 farther apart. 



Formation and Locality. — Limestone of Steeprock series, Steep- 

 rock lake, west-northwest of Lake Superior, Ontario, Canada. 



Atikokaxia irregularis, .\. SP. 



(PI. n. Fig. 1.) 



The specimen representing this species is a weathered, oblique 

 section, 6-5 cm. in height and 11 cm. in width. The radiating tubes 

 are more irregular and smaller than those of A. lawsoni, and the 

 general appearance of the specimen is more like that of a portion 

 of a large semi-globular sponge. 



A second frag-ment that may be referred to this species indi- 

 cates that the central cavity was very small. 



This species is associated in the same limestone with A. lawsoni. 



