TANvsirnoN. 201 



"The specimens received in spirits show that the mantle is 

 closed at the basal edge, presenting narrow openings at either 

 extremity for the passage of the siplional sheath and foot. The 

 suture is nearly straight, scarcely undulate. The foot is thick 

 and cartilaginous, broad, and -with a pit or depression at the 

 anterior end, and laterally hamate, or bent like the crook of a 

 walking-stick, the point being towards the hinge-side and the 

 hinder edge straight, not concave." 



324. Tanysiphon rivalis, Benson. 



Taivjtiiplton ricalis, l?ensoii, A. M. N. II. kit. :!, i, lSof<, p. 408, 

 pi. 12 B, tigs. 13; H. & T., C. \. 187(i, p. 47. pi. 116, 

 tigs. 1 c^' 4. 



Ori.f/lnal description: — Testa tenui, transverse oblonga, antice 

 breviori obtusa, postice longiori, angustiori, extus tenuiter striata 

 et irregnlariter remote rugosa, epidermide pallide cornea; natibus 

 decorticatis latis prominentibus. nmbonibus obtusis : valvis male 

 congrueutibus, per spatium breve cardinale et ventrale solum 

 tactum exhibentibus ; humero unibonali autioo breviter compressi- 

 usculo ; area interior! c.'erulescenti-alba. 



Long. 11, lat. 21, diara. 9 mm. 



Ilab. Streams in the neighbourhood of Calcutta. 



Family CYRENIDiE. 



Subfamily CYRENIN^. 



Shell equivalve, closing tightly, with umbones often eroded, 

 covered with a periostracum, not pearly, trigonal or roundly 

 ovate ; hinge usually bearing t\\"o or three cardinal teeth and both 

 anterior and posterior laterals ; ligament external, prominent : 

 pallial line simple or sinuous. 



Animal fluviatile. lacustrine or estuarine ; with anteriorly open 

 mantle: generally furnislied \^ith two siphons of variable size, 

 seldom a single (anal) one ; foot large, without byssus in the adult 

 state; palpes triangular; gills joined beliiiul, unequal, the outer 

 the shorter. 



Distribution. Asia; Africa; America; Australia. 



Genus CYRENA. 



Cyrena, Lamarck, Anim. s. Vert, v, 1818, p. 5ol. 



Ventts (pars), Cbemuitz, Martini &; Chemnitz, vi, 1782, p. .'>-33, 



pi. 32, fig. :5.",0. 

 Cyclas (pars), liriiguiere, Encycl. McHli. 1792. 

 Cyanoci/clns, Ft'nissac. Diet. Sci. Nat. xii. 1818. 

 Po/i/mexocla, Ilatinesque, Ann. (ion. Sci. I'hvs. et Xat. v, 1820, 



p. 219. 



