370 Transactions. * 



Genus Rhizothyris Thomson. 



Trans. N.Z. Inst., vol. 47, 1915, p. 399, figs. 5, a-d, 6, a, h. 



Specimens of Rhizothyris are extremely abmidant in the main Mount 

 Brown limestone (D), and less so in the uppermost limestone (E). Tliey 

 present a great variety of form, the extremes being so different that it is 

 impossible to imagine that they had not been differentiated into separate 

 true-breeding races, although there are so many intermediates that it is 

 obvious that the evolution either had taken place only a short time 

 previously or was still in progress. Similar polymorphism is displayed by 

 the specimens from Hutchinson's Quarry, Oamaru, the Maerewhenua green- 

 sands, and the Curiosity Shop. As the forms are not exactly the same in 

 these localities,, and as a stratigraphical value may be found -to attach itself 

 to certain forms, it is desirable to create species for all the distinptive 

 types. To show the interrelationships of these it will be necessary to 

 describe here a few shells from the other districts. The bearing of the 

 results on correlation is discussed in Part III of this paper. 



The ancestral type from which all the species appear to have developed 

 has not yet been found adult, but is represented by the half-grown shell 

 of R. curiosa Thomson. This is shown by the growth-lines to have been 

 a suborbicular shell with a broad, uncurved hinge-line, and without any 

 folding. From it development in outline apj^ears to have proceeded along 

 three main lines. 



The first series is characterized by a retention of the broad, uncurved 

 hinge-line, but there is an increasing elongation of the shell, combuied with 

 an increasing narrowing of the front. To this series belong R. curiosa 

 Thomson, R. media n. sp., R. scutum n. sp., R. rhizoida Hutt., and 

 R. elongata n. sp. 



The second series is also characterized by the retention of a broad, 

 little-curved hiage-line, and by increasing elongation, but this is not accom- 

 panied by a taperiag of the front, and the shells retain an elliptical shape. 

 Here belong R. curta n. sp. and R. elliptica n. sp. 



The third series is characterized by an increasing narrowing and curva- 

 ture of the hinge-line, and elongation is accompanied by a narrowing of 

 the front, so that the shape passes from subcircular through broadly ovate, 

 ovate, to narrowly ovate. Here belong R. lateralis n. sp., R 'pirum n. sp., 

 R. ovate n. sp., and R. amygdala n. sp. 



There has also been development in the amount of folding, and at each 

 stage in the development in outline folding may take place preventing 

 further development of outline in quite the same manner as would be 

 possible to an imfolded shell. The folding is in all cases simple ventral 

 uniplication (sulcate or concavi-convex of Buckman). In general the 

 effect of folding on outline is to produce a sudden truncation of the 

 front. The folded shells are also generally more convex than the un- 

 folded. Where necessary, species have been set up for the reception of 

 the strongly folded and convex forms — viz., R. crassa n. sp., R. obesa n. sp., 

 and R. fortis n. sp. 



Mr. S. S. Buckman has also pointed out to me that a further discrimina- 

 tion may be made according to the stage of foraminal developm^t. All 

 the species are permesothyrid, and almost epithyrid, but in some there has 

 been remigration of the foramen with the production of- pseudbtela. I am 

 not yet clear as to the specific value of this criterion, and have not applied 

 it in the present analysis. 



