[burwash] new species OF MARINE INVERTEBRATES 85 



eral slightly concave, with a projection along the hinge-line in the 

 central part, and the anteal, ventral and posteal margins form 

 a continuous curve, of which the ventral part is more broadly curved, 

 the anteal segment less so, and the posteal still more narrowly than 

 the anteal. There is also a slight suggestion of a rounded anteo- 

 ventral angle. The dorsal part of the posteal margin curves some 

 distance forward before its junction with the dorsal margin, whose 

 length is only slightly more than half of the total length of the speci- 

 men. The beaks are about 15 mm, distant from the anteal end. 



In each valve there is a sub-cardinal non-costate area, whose 

 lower edge extends in a convex curve from the beaks to a point on the 

 ventral part of the posteal margin, There is also an anteo-ventral 

 non-costated area, which extends also from near the beaks in a. crescent- 

 shape to a point near the posterior end of the ventral margin. The 

 zone lying between these two areas is crossed by eight short costae 

 which are curved backward in the middle. The sub-cardinal area has a 

 scimitar-shaped pit extending posteally from the beaks in each valve, 

 which represents the buttress anterior to the adductor scars. The 

 latter are well shewn, and crossed by two sets of ridges. The palliai 

 line also crosses the adductor scar from front to rear and is clearly 

 traceable throughout its extent. These are also Scabrœ — as shown 

 by the interior areal ridge — ^v^diich, however, is not very pronounced. 



Trigonia paucicostata (sp. nov.) 



(Plate III, Fig. 5) 



This is a small species represented by two interior casts of the 

 left valve, which are 21 and 25 mm. in length by 16 and 17 + mm. in 

 depth. The beaks are anterior in the smaller specimen and nearly 

 so in the larger. The sub-cardinal area has a median groove which 

 represents a ridge on the interior of the shell dividing the incurrent 

 and excurrent channels of the siphon, as in other Scabrœ. The re- 

 mainder of the valve is marked by five or six prominent costse. Those 

 of the posteal region are short and nearly parallel with the edge of the 

 sub-cardinal area, those further forward radiate from the umbonal 

 region and are curved downward and forward in their ventral ends. 

 They are not nodose and are traversed by concentric lines of growth 

 which are also visible crossing the area. 



The greatest convexity of a cast of one valve is 5 mm., and is 

 situated well forward. The outline of the valve is crescentic, and has 

 a somewhat regular curve extending from the beak throughout the an- 

 terior and inferior part of the margin, while the posterior end has a 

 somewhat narrower curve with a rounded angle at the intersection of 



