378 Transactions. 



Pachymagas parki (Hutt.). (Fig. 9; Plate XXV, figs. 8-11.) 



1905. Magellania parJci Hutt., Trans. N.Z. Inst., vol. 37, p. 476, 

 pi. xlv, fig. 4. 



1915. Pachymagas parki Thomson, Trans. N.Z. Inst., vol. 47, 

 • pp. 394-95, fig. 2, b (but not fig. 2, a). 

 The holotype (fig. 9) is a distinctly ovate form, but the majority of 

 specimens from the type locality have a similar broad but less curved 

 hinge-line, and the sides are less convex. The front is produced as a 

 narrow tongue. The beak is short and obtuse. The valves are moderately 

 convex, the ventral with a longitudinal fold which is rounded and obscurely 

 limited posteriorly, but anteriorly is more pronounced, flattened on top^ 

 and sharply limited laterally ; the dorsal valve is sulcate anteriorly and 

 sharply reflected to fit into the notch in the ventral valve ; the anterior 

 commissure shows a fairly narrow, deep, flat-bottomed sinuation. A 

 single specimen is known in which incipient ventral biplication is shown, 

 the anterior commissure presenting the form of a W with rounded angles. 

 The beak is erect^ not produced much beyond the hinge-line, with a 

 moderately large, mesothyrid, attrite foramen. 



Fig. 9.— Pachymagas parki (Kntt.). Holotype. Natural size. 



The cardinalia were described and figured by me in 1915. Attention 

 may be drawn to the prominent cardinal process, which is about half the 

 length of the hinge-trough, higher than the socket-ridges, and somewhat 

 swollen in front. Length of holotype, 37 mm. ; breadth, 29 mm. ; thick- 

 ness, 17 mm. 



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

 as above to strongly folded forms, the species is known only from the type 

 locality (where it is common), the tuffs of Whitewater Creek, Trelissick 

 Basin (a single specimen collected by McKay), and the main Mount Brown 

 limestone (where it is not very common). Two closely related species 

 have been differentiated — viz., P. McKayi and P. morgani. 



Pachymagas McKayi* n. sp. (Plate XXVII, figs. 1-3.) 



Shell ovate, beak fairly long, hinge-line moderately broad, strongly 

 curved, sides convex and regularly rounded, front somewhat produced. 

 Valves strongly convex, especially the ventral ; folding similar to P. parki, 



* Named after the late Mr. A. McKay. The International Rules demand that, for a 

 genitive, to the exact and complete name of a male an "i" should be added. The New 

 Zealand Geological Survey, following Hutton, have generally adopted the permissible 

 practice of commencing such names with a small initial letter, but this case shows the 

 disadvantage of this procedure. 



