300 



ZOOLOGY 



SECT. 



blind tubes receiving processes of the mantle ; the beak of the 

 ventral valve is prominent, and has a foramen partly bounded by a 

 deltidium of one or two pieces : there is a shelly loop springing 

 from the hinge-line of the dorsal valve. The genus Magellania is 

 characterised by having the shelly loop fully half as long as the 

 shell itself, and by the presence of a median septum on the inner 

 face of the dorsal valve. 



The specific differences between M. lenticularis and M. flavescens 

 are largely matters of detail, depending upon the precise form of 

 the shell and loop. More obvious differences are seen in the shell, 

 which is pink, evenly-rounded, and short-beaked in M. lenticularis, 

 while in M. flavescens it is horn-coloured, almost pentagonal, and 

 has a prominent beak. 



3. GENERAL ORGANISATION. 



The shell presents two distinct types : in the Articulata, the 

 order to which Magellania belongs, the dorsal and ventral valves 



FIQ. 300. Typical Brachiopoda. A. Lingula ; R, Crania ; C, Discina ; /;, Terebratula 

 E, Cistella ; F, Spirifera ; O, Kraussina. (After Bronn.) 



are dissimilar, the dorsal valve having a cardinal process and usually 

 a shelly loop, the ventral a spout-like beak for the peduncle ; while 

 in the Inarticulata, of which Lingula is a good example (Fig. 300, A), 

 the two valves are nearly alike, and there is no shelly loop and no 

 beak. These differences are accompanied by differences in micro- 

 scopic structure ; in the Articulata the shell is dense and stony, 

 and is formed of obliquely placed calcareous prisms, while in 



