270 DR. C. T. TRECllMAisnS' ON THE [vol. lxxix, 



seems undesirable at present to found a new genus until more 

 material is forthcoming. 



It seems to be individually variable, as are so many of the 

 bivalves in the Trias and Jura of New Zealand, as regards position 

 of the beaks, development of the posterior concave area and 

 of the sulcus on the surface of the shell, and in the degree of 

 inflation of the valves. 



' PsEUDOMONOTIS ' MARSHALLI, Sp. 110V. (PI. XV, fig'S. 6-9.) 



Description. — Shell thin, elongate, roughly oval in outline. 

 Beak of the left valve small, situated about the middle of the 

 hinge-line, and only projecting slightly above it, the hinge-line 

 being straight or slightly curved. Behind the beak is an obtusely 

 angular wing, the outline of which in some specimens is scarcely 

 differentiated from that of the shell-margin. The wing is devoid 

 of radiating ribs, but the concentric growth-ripples pass over it. 

 The lower posterior margin of the shell is produced, and is broadly 

 though well rounded. The lower margin is gently rounded, while 

 the anterior margin is produced and narrows somewhat, but is 

 well rounded. The surface of the left valve is gently rounded ; 

 that of the right valve is flatter, though still rounded. 



Numerous even and equidistant rounded ribs start from the 

 beak, and pass to the margins. At varying distances from the 

 beak secondary ribs are intercalated which pass to the margins, 

 but do not always attain the strength of the primary ribs. Seventy- 

 five ribs were counted round the margin of one specimen. Towards 

 the posterior wing and the posterior hinge-area the ribs become 

 weaker, and finally disappear. They are widest and most pro- 

 minent on the anterior portion of the valves, and gradually 

 decrease in size posteriorly. Growth-ripples occur near the beak, 

 and rather widely and irregularly spaced concentric ridges occur on 

 the body of the shell, tending to cut the ribs up into nodes. 



The beak of the right valve does not project appreciably above 

 the hinge -line. 



Dimensions. — The left valve (fig. 6) is 50 mm. long and 

 30 mm. high. 



Locality and horizon. — The slopes of Flag Hill in the 

 Hokonui Hills, in a fine-grained yellow sandstone. Callovian (?). 

 Several specimens of all sizes were collected, both left and right 

 valves. 



Remarks. — Sir James Hector 1 illustrates a left valve of this 

 shell, calling it ' PJioladomya (?)\ I can find no described species 

 resembling this form. Its rounded outline differentiates it 

 strongly from the species of Pseuclomonotis that occur in the 

 New Zealand Trias ; but, in the present state of nomenclature, 

 it must apparently be ranged under that generic name. 



1 ' Catal. Ind. & Col. Exhibition ' 1886, p. 69. 



