272 LECTURES ON MOLLUSCA. 



muscular impressions in the convex valve. The flat valve is pierced 

 by a hole, which is rilled up by a shelly plug, which is more or less 

 separate from the valve. The animal diners from the Oysters in hav- 

 ing a small foot, connected with the plug which takes the place of the 

 byssus in the mussels. The convex valve has four scars, of which the 

 largest is made by the plug muscle, and the front one by the adductor. 

 The third central scar, and one near the internal cartilage, are made 

 by the retractors of the foot. The Anomias are extremely thin and 

 pearly, found in all parts of the world, and in all ages from the oolites. 

 In Placunanomia, there are only two instead of three muscular scars. 

 The hinge fulcrum is notched, and the plug often becomes imbedded 

 in the lower valve. The fossil Limanomia is eared like Lima. Carolia 

 has a plug when young, like Anomia; but when adult it resembles 

 Placunopsis, and might be ranked with either family. It belongs to 

 the tertiary age. 



The species in this family ought always to be studied in connection 

 with their geographical relationships ; and the young animals ought 

 especially to be examined, as being less likely to be affected by the 

 disturbing influences of later life. 



CLASS PALLIOBRANOHIATA. 

 {Mantle-breathers, or Brachiopods.) 



The Palliobranchiate bivalves may be considered as a parallel group 

 with the Lamellibranchs, but inferior to them ; as the Implacental as 

 compared with the Placental Mammals. They are always attached, 

 either by the surface of the valve, or by a peduncle passing through a 

 hole, as in the Anomids. The resemblance however which caused 

 Linnasus to unite Terebratula with Anomia is only superficial. The 

 valves, instead of being side wings, are front and back shields. There 

 are no true ligaments or hinge teeth. Above all, there are no gills ; 

 the breathing being performed by the general surface of the skin. The 

 water-currents are established by the action of cilia and variously 

 twisted "arms," which gave Lamarck the class-name Brachiopoda. 

 But they are not, in any strict sense, arms or feet; not being used for 

 locomotion ; but on the contrary correspond to the lips of the Lamel- 

 libranchs, their office being to waft the food-particles to the mouth. 

 They are generally fixed to a shelly skeleton within, the form of which 

 is very characteristic of the genera. The valves of the Lamp-shells are 

 fastened by interlocking teeth ; but the work of ligaments is performed 

 by a set of muscles which act in the opposite direction from the adduc- 

 tors. After the skin and lips are deducted, the body of the animal 

 remains in but a small portion at the back of the shell, often parti- 

 tioned off by a strong membrane, in the centre of which is the mouth. 



As there is no special breathing organ, the mantle is more than usu- 

 ally supplied with blood vessels, and adorned with various filaments. 

 The marks of the blood vessels may often be traced in the valves of fos- 

 sil shells. These display far more of the peculiarities of the animal 

 than do the valves of Lamellibranchs, in which the hinge is almost the 

 only safe guide to their affinities. It is therefore fortunate that so 



