20 THE NAUTILUS. 



Of the fourteen species included in the *' trigona group" by Simp- 

 son in his excellent arrangement, seven are found in the Ohio River, 

 viz.: 



Q. ruhiginosa. Q. eoccinea. 



Q. trigona. Q. solida. 



Q. ohliqua Q. plena. 



Q. pyramidata. 



Of these eoccinea is quite different from any of the others, and 

 possibly does not belong to the genus at all. (Ortmann, Naut. 

 XXII, p. 10.) At any rate it is so entirely distinct from trigona, 

 undata and obliqua that it may be dismissed from further considera- 

 tion in this connection. The remaining species may be separated 

 into two very natural groups by a characteristic difference in the 

 form and position of the pseudo-cardinals. In his description of tri- 

 gonus. Dr. Lea states that the " cardinal (is) large, elevated and 

 widely clett in the left valve and emerges from a pit in the right 

 valve." This is absolutely correct and is one of the most character- 

 istic specific details. 



The pseudo-cardinal of the left valve is composed of two deltoid 

 teeth separated by a deep, triangular cavity for the reception of the 

 pseudo-cardinal of the right valve. At their upper extremity they 

 meet and in most cases are completely fused together so that they 

 might well be termed a single " widely cleft tooth " rather than dis- 

 tinct teeth. The anterior tooth is high and sharply beveled upwards 

 to a narrow, nearly rectilinear edge, which is nearly parallel with the 

 lower margin of the lunule. The cavity between them is deep and 

 comparatively narrow and extends obliquely backward and upward 

 until terminated by the fused extremities of the pseudo-cardinals. 

 The pseudo-cardinal in the right valve is triangular in shape, with a 

 sharp apex directed toward the beak. The posterior side is short and 

 nearly straight up and down ; the anterior side is much longer and 

 more oblique. On both sides are deep cavities for the reception of 

 the pseudo-cardinals of the left valve, and owing to the fusing of the 

 upper extremities of these teeth these cavities are continued entirely 

 around the pseudo-cardinal, which consequently appears to be 

 " emerging from a pit." 



The space between the pseudo-cardinal and lateral teeth (" inter- 

 dentum") is comparatively narrow. This is clearly shown on both 

 Barnes's and Lea's figures. This arrangement of the pseudo-car- 



