part 3] JURASSIC ROCKS OF NEW ZEALAND. 277 



A poorly-preserved Pecten, which may belong to this genus, 

 occurs in the Psiloceras Beds at Taylor's Creek, in the Hokonui 

 Hills. 



Pleuromya sp. 



Description.— Shell thin, beaks almost anterior, anteriorly 

 directed, tapering rapidly, close together and rather inrolled. 

 There is a sunken area in front and below the beak, and behind 

 it the hinge-area is much sunken. The lower margin is nearly 

 straight, the lower anterior margin is bluntly rectangular and 

 projects beyond the beak, the front margin is gently concave. The 

 shell is seemingly closed all round, presumably edentulous, and 

 seems slightly inequivalve, the left valve being apparently some- 

 what the larger. The surface is decorated with fine growth-lines, 

 and the whole surface is more or less irregularly furrowed con- 

 centrically, some specimens much more so than others. 



Dimensions. — Length = about 50 mm.; height = 35 mm.; 

 diameter of both valves = 30 mm. Another specimen is 40 mm. 

 long, 30 mm. high, and 25 mm. in diameter. 



Locality. — Totara Point, Kawhia. 



Remarks. — Several specimens of this Plenromi/a-Yike shell 

 were collected. The whole series seems to belong to one species, 

 despite the individual variation that has been mentioned. They 

 are much crushed in various directions and otherwise damaged, 

 and therefore it is not advisable to make any specific determina- 

 tion. One of them recalls somewhat a shell called ' Mactro- 

 inya (?) sp., : which Grottsche figures from the Lower Oolite of 

 Espinazito, in the Argentine Cordillera. 1 



Tkigonta kawhiana, sp. nov. (PI. XIII, figs. 6-9.) 



Description. — Beaks anteriorly situated, not very prominent, 

 flattened and compressed. The anterior margin slopes rapidly 

 down from the beaks and is gently rounded, the lower margin is 

 gently rounded, the posterior broadly rostrate and rather angulate 

 below. The areal margin behind the beaks is rather concave. A 

 slightly raised rounded ridge cut by the growth-lines into folia - 

 ceous nodes sweeps from behind the beak to the lower posterior 

 margin, separating the shell into an anterior gently rounded portion 

 and a flatter posterior areal portion. The latter has a faint furrow 

 passing down it, situated rather nearer to the areal margin than 

 to the ridge. The posterior surface is decorated with fine, rather 

 closely-set, equidistant, foliaceous growth-ridges. 



The main anterior part of the shell is ornamented with nine or 

 ten primary ribs that radiate downwards from the region of the 

 beak and from the anterior side of the ridge. These large primary 

 ribs are confined to the posterior part of the shell in front of the 

 ridge behind and below the beak, and only the last three or four 

 of them continue to the lower margin. In front of the beak 

 another series of finer but rather sharp raised ribs pass from the 



1 Of. cit. p. 33 & pi. vii, fig. 3. 



