Tno^isoy .—Geology of Middle Waipara and Weka Pass District. 371 



folding, and beak characters. It has, however, a much less advanced 

 cardinal process. Length of holotype, 35 mm. ; breadth, 31 mm. ; thick- 

 ness, 18-5 mm. 



Type locahty : Lower Mount Brown limestone (A), cuesta near Onepunga, 

 where it is the commonest species. 



Pachymagas haasti n. sp. (Plate XXV, figs. 4-7.) 



Shell elliptical, beak short, obtuse, hinge-line rather narrow, slightly 

 .curved, sides moderately convex, front slightly produced and gently rounded. 

 Valves moderately convex, the ventral with a broad rounded longitudinal 

 fold flattened anteriorly, anterior commissure with a moderatelv broad, 

 shallow, flat-bottomed, ventral sinuation. Beak nearly erect, little pro- 

 duced beyond the hinge-line, foramen moderately large, mesothyrid, attrite. 

 Cardinal process not more, than one-third the length of the hinge-line, 

 considerably less in height than the socket-ridges, rounded on the upper 

 front surface, Length of holotype, 37-5 mm. ; breadth, 30 mm. ; thick- 

 ness, 20 mm. 



This species resembles the elliptical forms of P. parhi in shape and 

 elongation and in beak characters, but is less strongly folded and possesses 

 a less advanced cardinal process. It may be regarded as ancestral to 

 P. parki. 



Type locality: Grreensands, Deborah, near Oamaru. The species is 

 common in most of the Hutchinsonian localities near Oamaru — viz., Devil's 

 Bridge, All Day Bay, Three Roads — but is rare in Hutchinson's Quarry. 

 It also occurs in the Clarendon limestone, the Ngapara limestone, and the 

 Maerewhenua limestone. It is rare in the main Mount Brown limestone. 



Pachymagas hectori n. sp. (Plate XXIV, figs. 10-13.) 



Shell broadly ovate, beak fairly short, hinge-line rather narrow and 

 curved, sides regularly rounded, front slightly produced, rounded. Valves 

 moderately convex, ventral valve with a broad rounded fold, flattened 

 anteriorly, dorsal valve with a broad, shallow, anterior sinus, anterior 

 commissure with a broad, shallow, flat-bottomed anterior sinuation. Beak 

 suberect, nearly erect, not produced far beyond the hinge-line, foramen 

 fairly large, mesothyrid, attrite. Cardinal process short, low, comparable to 

 that of P. haasti. Length of holotype, 39 mm. ; breadth, 33 mm. ; thickness, 

 21-5 mm. 



This species differs from P. haasti in the ovate outline, and from 

 P. 2)arl:i in the less advanced folding and cardinal process. 



Type locality : Greensands of Landon Creek, near Oamaru. The 

 species is common in most of the Hutchinsonian localities near Oamaru — 

 e.g. Rifle Butts,. Three Roads, Kakanui, All Day Bay, and Deborah — but 

 is rare at Hutchinson's Quarry. It also occurs in the Ngapara limestone, 

 and one specimen is known from the Maerewhenua greensands (loc. 179, 

 A. McKay). It occurs fairly commonly in the main Mount Brown lime- 

 stone, while a single specimen has been obtained from the uppermost Mount 

 Brown limestone. 



There is a further species from the main limestone which differs from 

 P. hectori chiefly in being narrowly and not broadly ovate. Unfortunately 

 none of the dozen specimens which I have collected are sufficiently well 

 preserved to serve as a type. 



