coxcnoLoor. 125 



and tliore are no distinct pallial orifices; in other genera, as Cardium, the orifices 

 of the niantl(! are ])rolonged into tubes, whicli in some cases are very extensile and 

 exceed the shell in length. The length of these tubes or siphons, as they are called, 

 may be estimated by the size and length of the siphoual scar on the shell, which 

 is always proportionate to that of the tubes themselves, and is longest in such 

 burrowing forms as Mya and ticrobicularia. Respiration is carried on by vascular 

 lamella; or gills, tiirough which the water taken in by the inhalent orifice is passed, 

 being at one and tlie same time dispossessed of its oxygen and such organic or other 

 matter as it may hold iu suspension, and which serve as nutriment for the animal, 

 and it is surjjiising how soon and how etfectually a group of lu'althy active bivalves 

 will clear a mass of turbid water (stained say with clay or any harmless piguu'iit) 

 by straining it through their siphons and gills. If any particle of a noxious 

 substance, however, is held in such a position as to be drawn into the inhalent orifice, 

 the itijured animal at once closes its valves with a snap, producing a violent jet of 

 water, with which the otfeuding substance is expelled. 



Locomotion is mostly performed by means of the foot, though some bivalves 

 progress by opening and closing their valves, as the ' Pectens,' and the foot is ahso 

 tlie organ by means of which Vjurrowing shells excavate their residence in rock or 

 wood, usually by means of the rugose surface of the valves, as in Pholas, which 

 being rotated by the muscular foot, rasp the rock like a file. In some cases, 

 liowever, the action would seciu to bo other than mechanical, and cither the result 

 of an acid secretion or perhaps merely the carbonic acid produced by respiration. 

 In the former case any substances less hard than the valves can be attacked and 

 perforated, iu the latter calcareous rocks only. 



Order ASIPHOXATA. 



Iles])iratory siphons none or rudimentary. Mantle lobes free. 



Many of this order are fixed by their shell and incapable of locomotion, save in 

 their earliest stage, as Odrca and Chama; others are moored by a bys.sus, as Mi/filus 

 and Pinna; whilst others, as Unio and Triyonin, move freely, the latter animal being 

 very active and capable of ltai>iug (when out of water) some distance. 



Faniihj Anomiidae. 



Byssus large and passing through a nearly complete foramen througli tlie right 

 mantle lobe and a hole in the lower valve. 



The shells of Anomin are thin, and often take the form of objects to which they 

 arc attached. The byssal plug is calcareous, and the upper shell has three muscular 

 scars. 



Anomi.v ^xigjiatica, Chetn. 



* ,, KLECTRiN.t, Chem. non L. 



* ,, Sol, lleeve. 



Family Placunidse. 



Shell subnacreous. Hinge formed of the divergent ridges. Shells free, buried 

 erect in sandy mud. 



Hinge ridges gradually diverge, the hinder much the longer. Shell semi- 

 transparent. 



*PL.\CEXT.i PLACENTA, L. 



This is the ' window oyster,' .so called from its valves being transparent like talc. 



Famihj Ostreidse. 



*0STEEA CUCTjLLATA, I?om. 



The 'Ladies slipper' oyster or 'Jinjouk' of the Burmese. This is the species 

 ■which crusts the surf-beaten rocks off the coasts of India and Burma, giving them 



