86 TBOCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



size nearly to the front edge of the visceral mass. This was naturally 

 taken to be a malformation, but examination of five adult specimens 

 showed the arrangement to be similar in all of them. 



The right ctenidium is continuous in all but one of the specimens. 

 Its laminae are larger than those of the left side and extend forward, 

 diminishing in size, between the palpi to a point opposite the middle 

 of the visceral mass. In the exceptional specimen there is a dis- 

 continuity of the laminae, but the vacant space of the left side is not 

 repeated on the right. The edge of the anterior segment of the inner 

 direct and reflected lamina of the right side is bifurcate, in front of 

 the anterior edge of the posterior segment. 



The fleshy septum above described completely separates the anal 

 and branchial chambers. The inhalent siphon opens into the latter 

 by a circular opening capable of being closed by a sphincter muscle or 

 arrangement of fibres. On either side, near this opening, is a large, 

 long sensory lamina, situated on the mantle and extending forward, 

 parallel with the thickened mantle-edge, nearly to the posterior edge 

 of the foot, where the latter joins the visceral mass. The ligament is 

 sunken, but not wholly internal ; it is short and small. The resilium 

 is not in contact with it, but is wholly internal, large, strong, and 

 mesially constricted. 



Zenatia Leshatesh, Reeve. 



This species is as inequilateral as the preceding, but in a reverse 

 direction, the longer end of the shell being the posterior, with a deep 

 pallial sinus. The ligament is longer than in jResania, and the 

 resilium rather smaller, but otherwise similar to that of Resania. 



The siphons are naked, and completely united ; but larger, laterally 

 more compressed, and less cylindrical than in Resania. The siphonal 

 openings are surrounded by conspicuous papillae. 



The mantle-edges are thickened, smooth, and solidly united in 

 front and forward on the ventral margin, half the length of the shell, 

 to the pedal opening. The sensory laminae occur on the inner side of 

 the mantle near the inner orifice of the inhalent siphon, but they 

 are smaller, shorter, and more delicate than in Resania. The foot 

 is quadrate, compressed, smooth, with entire edges; smaller, pro- 

 portionally, than in Resania. The mouth is circular, and the palpi 

 near it narrow and adherent to the surface of the body ; they are long 

 and large, their free ends extend behind the foot. The visceral mass 

 and siphonal septum are connected, but in this case there is no 

 fleshy septum between the ctenidia, but, as is more generally the 

 case in Pelecypoda, the bases of the ctenidia join to form the 

 partition between the anal and branchial chambers. The ctenidia 

 are symmetrical, more coarsely plicate than in Resania, and extend 

 forward, diminishing in size, a considerable distance between the palpi. 

 There is no discontinuity ; the discrepancy in size between the inner 

 and outer laminte of each ctenidium is less marked than in Resania. 



The heart, visceral organs, etc., do not present in either genus 

 any distinctive peculiarities compared with other members of the 

 Mactracea. 



