234 Transactions. 



The description is derived from two right valves, and the measurements 

 from the larger of the two. It is a minute form compared with others 

 recorded from the New Zealand Tertiaries. 



Mr. Murdoch was good enough to write this description. 



Trigonia areolata n. sp. (Plate XV, fig. 1 ; Plate XVII, fig. 1.) 



Shell small, ovate, inequilateral, slightly extended posteriorly. One 

 valve 14 mm. high and 15 mm. long ; another 17 mm. high and 18 mm. 

 long. Interior of the shell highly nacreous. Beaks rather prominent, 

 distinctly prosogyrous. Dorsal margin descending rapidly in front, and 

 posterior margin evenly rounded ; posterior ventral margin nearly circular ; 

 anterior margin at first well rounded, then ascending gradually to the 

 umbo. Sculpture a series of broad rounded ribs except over the area. 

 There are fifteen of these ribs, which are distinctly broader than the 

 grooves which separate them. Grooves crossed occasionally by irregular 

 fine lines of growth. Ventral margin slightly crenated. Near the ventral 

 margin on the interior surface the grooves have a few irregular stria- 

 tions in the direction of their length. The posterior '' area " constitutes 

 about one-fourth of the surface of the valve. Close to its border it has 

 one rib-like elevation extending from the posterior margin one-third of 

 the distance to the umbo. Another much broader rib near the posterior 

 margin of the area, also extending about two-thirds of the distance to the 

 umbo. A series of fine transverse ridges cross the area. The right valve 

 shows one large cardinal tooth, which, however, shows no trace of striation 

 The posterior part of the hinge has not been preserved. 



Two right valves only, both with somewhat incomplete hinge. The 

 nacreous internal surface and the posterior area appear to make it neces- 

 sary to refer this shell to the genus Trigonia. The smooth tooth, however, 

 as well as the prosogyrous character, are most anomalous. Suter suggests 

 that there may be a subgenus of Trigonia possessed of this peculiarity, 

 but there is no mention of this in the available literature. Cossmann in 

 his work Sur V evolution des Trigonies does not mention a single species 

 which has these two characters. 



Trigonia densicostata n. sp. (Plate XVI, fig. 1.) 



Shell rather large, 50 mm. high and 65 mm. long. Interior surface 

 highly nacreous. Umbo indistinct but apparently pointed forward. Dorsal 

 margin nearly straight, but soon descending and sweeping with a rounded 

 curve to the ventral margin, which is not complete but appears to bend 

 round evenly to the posterior margin, which is also incomplete. Sculpture : 

 A large number of fine, rather sharp radiating ridges which are rather 

 narrower than the intervening grooves. The ribs are nearly equal, and 

 they can be distinctly seen on the interior svirface of the shell. Hinge-line 

 incomplete, but with one prominent pointed tooth somewhat resembling 

 that in T. areolata Marshall, described above. 



The generic position of this shell is very doubtful. In my former list 

 it was referred to Cardium (Papyridina) , but Suter afterwards referred it 

 with the greatest hesitation to Pseudomonotis. It is described here merely 

 in order that it may be possible to refer to it by name, but it will shortly be 

 forwarded to Mr. H. Woods for identification. In the meantime it may be 

 said that the species does not belong to any other genus of Tertiary or 

 Recent mollusca hitherto found in New Zealand. 



One specimen only, somewhat imperfect. 



Type in the Wanganui Museum. 



