374 Transactions. 



Rhizothyris amygdala n. sp. (Plate XXII, fig. 2; Plate XXIII, fig. 4.) 



Shell narrowly ovate or almond-shaped, beak rather short, hinge-line 

 short and curved, sides convex and rounded, front narrowly truncated 

 and gently rounded. Valves moderately convex, sinuation of the anterior 

 iuargin broad and shallow. Length of holotype, 29'5 mm. ; breadth, 21 mm. ; 

 thickness, 14 mm. 



Type locality : Greensands of Hutchinson's Quarry, Oamaru. No 

 other specimens than the holotype are known. Two anteriorly imperfect 

 specimens from the main Mount Brown limestone agree closely in shape, 

 except that the beak is longer. 



Genus Pachymagas Iher. 



As in the case of Rhizothyris, specimens of Pachymagas are abundant 

 in the main Mount Brown limestone (D), and they present an even greater, 

 variety of characters. Similar .series occur in the Hutchinsonian green- 

 sands of Hutchinson's Quarry, Deborah, Kakanui, and other localities 

 near Oamaru, while there is a different but even more varied assemblage 

 in the Curiosity Shop greensands. The complexity of the problem, 

 combined with the large amount of material, has delayed the com- 

 pletion of my memoir on the Tertiary Brachiepods of New Zealand ; 

 in the present paper I have restricted myself to the description of the 

 majority of the species that can be recognized in the Waipara area, leaving 

 a few till more satisfactory material can be obtained. 



Among the already-described species in the Oamaruian three series may 

 be recognized. The first includes P. parki (Hutt.), P. marshalU (Andrew), 

 and P. trelissickensis Thomson, and probably also the Patagonian forms 

 P. tehuelcha Iher. (the genotype) and P. gigantea Ortmann, and the 

 Antarctic form P. antarctica Buckman. The shells are large, with a 

 prominent mesothyrid foramen and beaks which are suberect to erect and 

 not markedly carinatQ. The second series includes P. triangularis (Hutt.), 

 P. huttoni Thomson, and the Patagonian form P. venter Iher. The shells 

 are also large, with large mesothyrid foramens, with erect to incurved beaks 

 which are carinate, and with flattened and broadly sinuated dorsal valves. 

 The third series, perhaps generically distinct, includes P. ellipticus Thomson, 

 a smaller shell with a small foramen which is submesothyrid, almost 

 mesothyrid.* The majority of the specimens from the district belong to 

 the P. parki series, while there is a fourth series represented by a new 

 species, P. andrewi, with a large shell and a small foramen. 



In Neothyris, which is a descendant of Pachymagas, the tendency is to 

 increasing convexity, a restriction of the foramen, and the production and 

 incurving of the beak with old age, so that one may conclude that in 

 Pachymagas also the evolutionary process is from depressed to convex, 

 from large to small foramen, and from suberect to erect and produced 

 beak. Other characters which may be treated on evolutionary lines are 

 the shape, the folding, dnd the cardinalia. The last, unfortunately, are 

 rarely av&,ilable in specimens from the district, owing to the hardness of 

 the matrix. 



Pachymagas parki Series. 



The species may be arranged according to shape in a series from sub- 

 orbicular to narrowly ovate, but do' not all form a strictly phylogenetic 

 series, owing to variations in the amount of folding. 



* Cf. p. 380. 



