166 DISCOVERY REPORTS 



costata typical and filholi occurred in the same dredging with the subspecies here 

 described. 



Genus Cratis, Hedley, 19 15 

 Type (original designation) : Cratis progressa, Hedley, Recent, New South Wales 

 Cratis retiaria, n.sp. (Plate XLV, fig. 4). 



Shell small, white, solid, subquadrate, inequilateral; beaks near anterior end. 

 Prodissoconch of moderate size, having a broad well-defined rim and the middle raised 

 to a sharply pointed boss. Anterior margin almost vertical, practically at right angles 

 to the straight hinge line. Posterior margin fairly straight, nearly vertical medially, but 

 rounded off to the hinge plate, and gently curved into the broadly rounded sweep of the 

 basal margin. Sculpture of fairly strong closely spaced concentric ridges, decussated 

 by equally strong and more numerous radials. The radials number about fifty-two, and 

 the concentric ridges about twelve per millimetre, near the margin. The sculpture 

 resolves into a regular pattern of very numerous oval to rectangular raised granules. 

 Hinge typical : chondrophore broadly triangular, oblique, situated immediately below 

 the middle of the prodissoconch. Upper margin of hinge plate crowded with vertical 

 striations. Secondary teeth strong, situated distally, below the striations. Left valve 

 with two anterior vertical teeth, the innermost much the stronger, and three posterior 

 transverse teeth, the middle one of which is strongest. Margin of valves bevelled and 

 without crenulations. 



Length 2-5 mm. ; height 3 mm. ; thickness (one valve) 0-9 mm. (Holotype). 



Habitat: Off Three Kings Islands, St. 933, 260 m. 



This species is nearest allied to the lower Miocene (Tutamoe Series) C. ovata, 

 Marwick, 193 1 (N.Z. Geol. Surv., Pal. Bull., 13, p. 60), which is the only previous record 

 of the genus from New Zealand. 



The two species here described add this genus to the New Zealand Recent fauna. 



Cratis delicatula, n.sp. (Plate XLV, fig. 3). 



Shell small, white, moderately strong, subquadrate, inequilateral ; beaks near anterior 

 end. Prodissoconch small, cap-shaped, with a well-defined rim and a sharply raised 

 boss at the middle. Shaped very like retiaria but narrower. Sculpture of fine and 

 numerous closely spaced concentric ridges crossed by thin radials. These radials are 

 arranged in two series, a few widely spaced ones over the anterior area of the shell, and 

 finer, much more numerous and closely spaced ones over the posterior area. Hinge 

 fairly typical; chondrophore narrow, descending obliquely, posteriorly, from im- 

 mediately below the umbo. Upper margin of hinge plate crowded with vertical 

 striations. Secondary teeth strong, situated distally upon the hinge plate, below the 

 striations. Right valve with one strong anterior vertical tooth shaped like an inverted 

 L, and three posterior transverse teeth. In the left valve there are two anterior teeth 

 which fit on either side of the inverted L tooth of the opposite valve. On the lower part 

 of the posterior margin there are a few faint crenulations. 



