2 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



large and deep. The siphon-tubes and posterior pallial tentacle are 

 long. The palpal tentacles are long and tapered ; in life they may 

 extend nearly to the end of the expanded siphon." 



SHELL. 

 FIGURES 1 AND 2. 



To the above characters of the shell may be added a few others, 

 part of which, no doubt, are common to all the species of the genus, 

 while others are specific. 



When the animal is taken from the mud in which it lives, the 

 anterior portion of the shell is jet black. Posteriorly it gradually 

 assumes olive green. The black is no doubt a stain, as it bleaches 

 out in animals kept in aquaria, and the shell assumes a rather uni- 

 form olivaceous tint, somewhat streaked along some of the more 

 prominent lines of growth with yellow or brown. 



On each valve, two more or less pronounced radial stripes extend 

 from the beak to the ventral margin, one anterior, the other posterior, 

 Fig. 1. The margin of the mantle opposite the extremity of each of 

 these stripes is specially modified and sensitive. Internally, near the 

 dorsal margin of each valve, are two rows of toxodont teeth, one 

 extending anteriorly and the other posteriorly from the cartilage pit, 

 Fig. 2. Near the cartilage pit these teeth are very small and closely 

 placed. They attain their greatest length about midway along each 

 row, and become short and rather widely separated at the ends 

 furthest from the cartilage pit. 



Internal markings are rather obscure. Besides the scars of the 

 foot and adductor muscles, and the pallial line with its deep and 

 broad sinus, there is on each valve a rather distinct, curved line, ex- 

 tending from the ventral margin of the scar of the anterior adductor 

 muscle, nearly to the cartilage pit. Dissection shows that this line 

 marks the limit of the genital mass and digestive gland. 



Tryon* refers to this mark on the right valve, and thinks it is 

 caused by the loop of the intestine that extends forward, on the right 

 side, very near the shell. That this view is wrong is shown by the 

 presence of similar markings on both valves. 



* Structural and Systematic Conchology. 



