376 Transactions. 



attrite ; the beak-ridges are sharp. Length of holotype, 48 mm. ; breadth, 

 43 mm. ; thickness, 23 mm. The species differs from P. marshaUi only in 

 slightly greater elongation. 



Type locality : Sands below main Mount Brown limestone (D), ciiesta 

 overlooking the Weka Pass. 



Smaller specimens referable to this species occur in the second creamy 

 calcareous sandstone overling the main Mount Brown limestone, in the 

 Weka Pass stone at Onepunga, in the Oamaru district in the greensands 

 of Landon Creek, Three Roads, Kakanui, All Day Bay, and Hutchinson's 

 Quarry, and in the Caversham sandstone (A. McKay, loc. 309). 



Pachymagas bartrumi n. sp. (Plate XXIV, figs. 7 9.) 



Shell elongate, beak fairly short, obtuse, hinge-line nearly as broad as 

 the shell, little curved, sides very gently convex, front slightly narrowed, 

 rounded. Valves moderately convex, ventral valve bluntly carinate 

 longitudinally, anterior commissure with a broad, shallow, flat-bottomed 

 ventral sinuation. Beak nearly erect, foramen large, mesothyrid, attrite. 

 Length of holotype, 48 mm. ; breadth, 38 mm. ; thickness, 25 mm. 



This species represents a further stage, and, so far as is known, the 

 extreme stage, of elongation of the series P. marshaUi, P. cottoni, but has 

 an even broader hinge-line relative to the breadth of the shell. 



Type locality : $oot of dip-slope, main Mount Brown limestone, Weka 

 Pass. The species is rare in the type locality, and is not known elsewhere. 



Pachymagas speighti n, sp. (Plate XXV, figs. 12-14.) 



Shell shield-shaped, with a rather short obtuse beak and a broad nearly 

 straight hinge-line, whence the sides at first curve gently outwards to the 

 middle of the shell and then taper quickly to a narrow produced front. 

 Dorsal valve rather flat, reflected anteriorly, ventral valve moderately 

 convex, bluntly carinate longitudinally, anterior commissure with a narrow, 

 fairly deep, flat-bottomed sinuation. Beak nearly erect, little produced 

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

 of holotype, 41 mm. ; breadth, 37 mm. ; thickness, 21 mm. 



This species, which agrees with the above three in the possession of a 

 broad, straight hinge-line, differs from them in the narrowing of the front 

 and the more pronounced folding. All the succeeding species possess 

 narrower, and in general more curved, hinge-lines. 



Type locality : Foot of dip-slope, main Mount Brown limestone. The 

 species is rare in the type locality. A well-preserved cast, collected by 

 Professor Park from the Deborah limestone, agrees closely in outline, but 

 evidently belonged to a more convex shell. 



Pachymagas clarkei n. sp. (Plate XXVI, figs. 3-6.) 



Shell broadly elliptical, with a short, obtuse beak, a hinge-line of 

 moderate breadth but nearly straight, gently curving sides and a shghtly 

 produced front. Dorsal valve flattened, and sharply bent anteriorly to 

 fit the notch in the moderately convex, bluntly carinate ventral valve, 

 anterior commissure with a deep flat-bottomed sinuation of moderate 

 breadth. Beak erect, little produced beyond the dorsal valve, with a larg6 

 mesothyrid, attrite foramen. Cardinal process short, narrow, sharp, little 

 more than half the height of the hinge-trough. The species is shorter 

 and broader than P. farki, but otherwise resembles it in shape, convexity, 



