1901.] NATURAL SCIENCES OF PHILADELPHIA. 401 



ble anywhere, even with a lens. Beaks somewhat prominent, 

 small and in contact. Anterior end rounded, the slope above 

 straight; posterior slope straight or slightly convex, finely serrate; 

 the posterior end narrowly subrostrate and biangular, the right 

 valve having two prominent posterior keels, the space between them 

 concave, left valve with one posterior keel, a narrow furrow close 

 before it, with a slighter second depression, the basal margin well 

 rounded, ascending and sometimes slightly sinuous behind. Lunule 

 lanceolate, very deeply cut, bounded by acute ridges, that of the* 

 right valve rising well above the left, and with a wider excavation. 

 Area also deeply excavated, bounded by keels, the ligament promi- 

 nent. Interior white, the hinge strong, with two cardinal teeth in 

 each valve, the left anterior tooth and the right posterior bifid. 

 Left valve without laterals, right valve with low, distant anterior 

 and posterior lateral teeth. Hinge-line straight behind the beak, 

 concave in front. Pallial sinus very large, reaching to within a 

 millimeter or two of the anterior adductor scar, confluent with the 

 pallial line below for about half its length. Scars of the cruciform 

 muscle distinct. 



Length 38, alt. 29.5, diam. 11.5 mm. 



Inland Sea of Japan. Types No. 71,029 Coll. A. N. S. P. 



This species is closely related to T. pristis Lam. and T. concen- 

 trica Old. It has a wider lunule than the former, its bounding 

 keels without the irregularity, ' ' saw ' ' or serration seen in T. 

 pristis. The posterior area is more deeply excavated, the posterior 

 keel of the right valve is stronger, and the end is much more bent 

 to the right. The hinge-plate is wider, and the anterior lateral 

 tooth is further removed from the cardinals. Finally, the dorsal 

 slopes are steeper, meeting at a smaller angle, and hence the whole 

 outline is more triangular. In T. concentrica Gld. (Fiji Islands) 

 the form is more elongate, the lunule and posterior area far less 

 impressed, and the interior is more glossy, with shallower, less dis- 

 tinct muscular scars, and the shell is thinner. T. diaphana Desh. 

 differs by having the pallial sinus abut against the anterior adduc- 

 tor scar, according to Deshayes' description. T. siamensis v. 

 Martens is a longer, less high species, by the description. It has 

 not been figured, so far as I can learn, and is doubtfully distinct 

 from T. diaphana Desh. 



26 



