SUB-GENUS HETEROTRYPA. 143 



inclination to reach the surface on one side or the other. In the 

 vertical portion of their course, the corallltes are thin-walled, 

 and their walls are wavy or are sharply undulated from side to 

 side, and I have not been able to detect any tabulae in this 

 region. In the outer part of their course, on the other hand, 

 the walls are somewhat thickened, while a moderate number 

 of tabulae are now developed. There is mostly no difference 

 observable in the tabulation of the corallites, though here and 

 there a small-sized tube with close-set tabulae may be detected ; 

 and the tabulae are in all cases complete and approximately 

 horizontal. 



The only two species of MonticiUipora, known to me, with 

 which the present form could be confounded, are AL uiam- 

 fnulata, D'Orb., and I\I. moles fa, Nich. ; and the differences 

 in its minute structure, as above described, are so marked as 

 to render it unnecessary to compare it in detail with either of 

 these types. 



I have named the species in honour of my friend Principal 

 Dawson of Montreal, whose name has been so long and 

 so honourably associated with the investigation of Canadian 

 geology and palaeontology. 



Horizon and Locality. — Rare in the Cincinnati Group, 

 Ohio. 



Monticulipora (Heterotrypa) Jamesi, Nich. 

 (Figs. 25 and 26.) 



Chcetetesjavtesi^ Nicholson, Quart. Journ. Geol. Soc, vol. xxx. p. 506, PI. XXIX. 

 figs. 10-10/;, 1874. Pal. Ohio, vol. ii. p. 200, PI. XXL figs. 11, \\a^ 

 1875. Ann. Nat. Hist., ser. 4, vol. xviii. p. 89, PI. V. fig. 5, 1876. 



Spec. Char. — Corallum dendroid, the branches varying in 

 diameter from about a fifth of an inch up to half an inch, 

 dividing dichotomously, terminating in rounded free ends, 

 and sometimes becoming palmate by partial fusion. Calices 

 oval or rounded, sometimes obtusely indented on one or more 

 sides, thick-walled, from i-yoth to i-6oth inch in diameter. 



