232 Description of the Plates. 



FlGUllE 2. 



Lateral view of viscera of Brachiopod, Rhynchonella Psittacea, together with portions 

 of the mantle, peduncle, and arms; after Huxley, Proc. Roy. Soc, 1854. 



The view here given shows that a section of the animal in the 

 plane of the aperture of its two valves would not divide it into two 

 symmetrical halves, as in the Lamellibranchiata, and that its 

 valves consequently cannot be spoken of as right and left, as in 

 those animals. The two valves of the shell, the posterior third 

 only of the cavity of which was occupied by the body of the animal, 

 have been removed, as have also the anterior two-thirds of the 

 mantle lobes which were in relation with, though they did not, 

 as in Terebratula, send coecal processes into them. The spirally 

 coiled mass which the arms made up, and which filled up the 

 greater part of the mantle cavity, has been removed ; the arm of 

 the right side has been cut away from its origin, and only a small 

 part of the arm of the opposite side left on the other side of the 

 mouth. 



lis. Lobe of mantle v/hich was in relation with the valve ordi- 

 narily called ' dorsal.'' 



Mi. Lobe of mantle which was in relation with the so-called 

 ' ventral' valve. Both valves in the Braehiopoda arise from 

 the ' dorsal ■' or ' haemal' aspect of the animal, as they do in 

 the Lamellibranchiata, but in the former class they are 

 articulated across, whilst in the latter they are articulated 

 alonff the dorsal ridge. It is the ^ventral' valve which, 

 either by becoming adherent to some marine object, as in 

 Thecidium and Crania, or, as in the species before us, by 

 giving attachment on its inner surface to the peduncle p, 

 furnishes the entire shell with a fixed point for its limited 

 movements in the adult state. And Braehiopoda, whilst 

 still free in certain stages of their development, have been 

 observed to move from point to point by means of a pair 

 of rigid spines, one of which was inserted on either side of 

 the ventral pallial lobe. During the free stages of the life 

 of such larvae the dorsal valve, as being the heavier, has 

 been ordinarily observed to be inferior in position; it is so 



