236 DE. C. T. TRECHMAXX OX [voL 1x1 1 



rapicUv- widening, triangular sulcus ; but towards the anterio] 

 margin both ridges and sulcus become Aveaker, and merge into the 

 rounded surface of the shell. The junction of the shell is but 

 slightly sinuous. 



The type-specimen is 51 mm. long, 63 mm. wide, and the two 

 valves are 23 mm. deep. 



Locality and horizon. — Casts apparently of this species 

 occur in Carnic and Xoric strata at Otamita, Hokonui Hills. At 

 Nugget Point it is extremely abundant, together with Bpiriferina 

 (liomedea, at the southern end of Eoaring Bay, in a bed of very 

 hard pebbly sandstone of Khaitic age, which here closes the 

 Triassic sequence. It is less common at Kawhia, where it occurs 

 sporadically through a very thick series of grey sandy Eha^tic 

 deposits overlying the Xoric Pseiidomonotis shales. I obtained 

 one very fine example there and several less perfect ones, also 

 several specimens at Xugget Point. The ]S[ew Zealand Greological 

 Survey possesses a specimen from blue sandstones and chert at 

 the main branch of Taylor's Creek, on the south side of the 

 Hokonui Hills, in beds apparently of Ehsetic age. 



Hectoeia TrMiDA, sp. nov. (PI. XXY, fig. 2.) 



Shell very slightly wider than long ; valves inflated, the ventral 

 often rather more so than the dorsal. The hinge-line slopes gently 

 from the beaks, and represents rather less than the greatest width 

 of the shell ; the area is concave, and striated parallel to the margin. 

 The ventral beak is broad, and projects slightlj' above the hinge- 

 area. The delthyrium is triangular and insunken, and closed 

 internally by shelly matter ; it is partly concealed by the dorsal 

 beak. The outline is gently rounded at the sides, but is some- 

 what prolonged and rounded anteriorly. Eounded ridges diverge 

 in both valves from the beak, and extend to the anterior lateral 

 margins ; but in this species they become somewhat indistinct, and 

 tend to merge into the swollen and rounded outline of the shell,, 

 and on the dorsal valve they form a more or less flattened or gently 

 rounded, raised, median, triangular fold rather than a sulcus. The 

 growth'lines are well marked towards the anterior margin. 



A typical specimen is 60 mm. long and 63 mm. Avide. 



Locality and horizon. — Benmore Cutting, on the south 

 side of the Hokonui Hills, in a coarse, decomposed, pebbly,, 

 felspathic sandstone. Many casts of this large shell occur here, 

 I collected and brought home several internal casts, and a mould 

 from which I was able to obtain a gutta-percha impression of the 

 A'entral beak and ai-ea and dorsal valve; but the surface of the shell 

 is damaged and pitted by the quartz-grains in the rock. The 

 description of the internal structure of the hinge-region given in 

 the diagnosis of the genus is based on moulds of the separated 

 ventral and dorsal valves, which I obtained in this locality and 

 brought home with me. Rh?etic. 



He marks. — This is a distinctive form, and seems to occur in 

 hiffh Rhsetic beds. It differs from H. hi sulcata, in its lars^er size 



