132 Transactions. 



thyrid stage may be detected, but the condition of the foramen is telate, 

 for the ends of the ridges are not worn ofi. But in Rhizothyris, which Dr. 

 Thomson has, on internal characters, separated from Bouchardia, the beak 

 becomes an external tell-tale. According to his figure of R. rJdzoida* 

 the foramen is mesothyrid attrite. Therefore, in position the foramen of 

 Bouchardia is more advanced than that of Rhizothyris, but in condition it 

 is less advanced. These features indicate that these forms are not lineal 

 relatives, but are collaterals. The precedent beak-stages which may be 

 expected in some ancestor of both genera would be mesothyrid telate ; but 

 the divergence may have begun earlier.| 



The foraminal positions may be diagrammatically illustrated by rules 

 and circles in the following manner, where the rule stands for the line of 

 beak-ridges and the circle for the foramen : — 



(5.) Epithyrid Q 



(4.) Permesothyrid — — — O- 



(3.) Mesothyrid O- 



(2.) Submesothyrid q- 



(1.) Hypothyrid q 



These notes do not exhaust the subject, even for Terebratuloids. A 

 fuller discussion of these and of other development phases is in type for 

 the memoir in the " Palaeontologia Indica." It is hoped, however, that 

 these preliminary remarks may be of service until that work appears. 



Art. XIV.^ — High-water Rock-platforms : A Phase of Shore-line Erosion. 

 By J. A. Bartrum, University College, Auckland. 



[Read before the Geological Section of the Wellington Philosophical Society, 20th October, 



1915.] 



Plate'^iXIII. 



To most geologists, and certainly to all visitors to the historic Bay of 

 Islands, the " Old Hat " needs no introduction. It has been carved from 

 an emergent knob on a drowned spur, and, as the illustration (Plate XIII) 

 shows, is most aptly named. It was first brought into scientific prominence 

 by Professor J. D. Dana, J who visited the district in 1840 as a member 

 of the United States Exploring Expedition of 1838-42. 



* Trans. N.Z. Inst., vol. 47, 1915, p. 398, fig. 5a. 



t According to specimens of Rhizothyris rhizoida very kindly sent by Dr. Thomson, 

 the foramen is pormesothyrid attrite. This slightly modifies the above statement, but 

 does not invalidate the argument. — 2nd March, 1916. 



I J. D. Dana, Unit. States Explor. Exped. 1838-42, vol. 10, Geology, p. 109, 1849. 



