Thomson. — Geology of Middle Waiparaand Weka Pass District. 371 



Rhizothryis curiosa Thomson. (Plate XXII, fig. 3 ; Plate XXIV, fig. 5.) 

 Trans. N.Z. Inst., vol. 47, 1915, p. 399, fig. 6, a, h. 

 Three specimens referable to this primitive species have been found in 

 the uppermost limestone (E). It occurs also in the Curiosity Shop green- 

 sand, the Ngapara limestone, and the Clarendon limestone. 



Rhizothyris media n. sp. (Plate XXII, fig. 4 ; Plate XXIV, fig. 3.) 



Similar to R. curiosa in shape and size, but slightly more elongate. 

 The growth-lines repeat the outline of R. curiosa at about three-quarters 

 the length of the dorsal valve, and the subsequent development is in the 

 direction of^a narrowing front, making the outline shield-shaped instead of 

 suborbicular. The sides curve outwards only very slightly from the broad, 

 nearly straight hinge-line. The convexity, folding, and beak characters 

 are essentially similar to those of R. curiosa. Length of holotype, 38*5 mm. ; 

 breadth, 35 mm. ; thickness, 18 mm. 



Type locality : Curiosity Shop, Rakaia River, Canterbury. Besides 

 the type locality, this species occurs in the limestone of Fossil Point, 

 Ashburton River (Haast coll.). One specimen from the lower shell-bed 

 of the Weka Pass Stream and several from the uppermost Mount Brown 

 limestone (E) are referable ^here. 



Rhizothyris scutum n. sp. (Plate XXII, fig. 10 ; Plate XXIV, fig. 2.) 



Shell in outline resembling a long herg,ldic shield. The hinge-line is 

 nearly the breadth of the shell, and only slightly curved. The growth- 

 lines repeat the outline of R. scutum at about three-quarters the length 

 of the dorsal valve. The shell is very little folded, and both valves are 

 rather depressed, as in R. curiosa and R. scutum. ' Length of holotype, 

 43 mm. ; breadth, 34 mm. ; thickness, 19*5 mm. 



Type locality : Greensands, Hutchinson's Quarry, Oamaru. Besides 

 the .type locality, this species occurs also in limestone near Clifden, Waiau, 

 Southland (G. M. Thomson coll.), in the Curiosity Shop greensands, Rakaia 

 River, and in the main Mount Brown limestone (D) and the uppermost 

 limestone (E) of the Weka Pass. 



Rhizothyris rhizoida (Hutt.). (Plate XXII, fig. 6 ; Plate XXIII, fig. 7.) 



1905. Bouchardia rhizoida Hutt., Trans. N.Z. Inst., vol. 37. p. 480, 



pi. xlvi, fig. 7. 

 1907. Magellania rhizoida Iher., Ann. Mus. Nac. Buenos Aires, 



ser. 3, tom. 7, p. 473. 

 1915. Rhizothyris rhizoida Thomson, Trans. N.Z. Inst., vol. 47, 

 pp. 397-99, fig. 5, a. 

 • The holotype, which comes from the Weka Pass, is a shell with worn 

 beak, which led Buckman from an examination of the figure to state that 

 the foramen was mesothyrid, but it is really perinesoth}Tid. Shells exactly 

 matching the holotype in shape are not common, but the species may be 

 allowed to include those shells intermediate in elongation between R. scutum 

 and R. elongata which are moderately elongate, with a slightly curved hinge- 

 line nearly the breadth of the shell, and a marked taper. These shells are 

 moderately to strongly convex, and always show some folding. 



The species is common in the main Mount Brown limestone (D), from 

 which the holotype was doubtless derived, but does not appear to extend 

 into the uppermost limestone. It is also common at Hutchinson's Quarry. 



