viii PHYLUM MOLLUSCOIDA 367 



Systematic position of the Example. 



The genus Magellania, of which there are several species, 

 belongs to the family Terebratulidse, and to the order Articulata. 



The dissimilar valves of the shell articulated by teeth and 

 sockets, and the absence of an anus, place it among the Articulata. 

 The Terebratulidas are distinguished by an oval or rounded shell, 

 the structure of which is punctate, the dots corresponding with 

 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 

 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. 294, 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 

 the Inarticulata it has no prismatic structure, but usually con- 

 sists of a chitinoid material more or less strengthened by calcareous 

 spicules. Among the Articulata the loop may be absent ; when 

 present, it varies greatly in form and size, being sometimes very 

 small and simple (Fig. 294, C, D), sometimes bent upon itself, as 

 in Magellania, sometimes attached to the septum or to the interior 

 of the dorsal valve (E), sometimes, as in the extinct Spirifera, 

 represented by a complex double spiral (F), sometimes reduced to 

 short, paired rods springing from the septum (G). 



The majority of both orders are attached by a longer or shorter 

 peduncle which passes between the proximal ends of the valves in 

 Lingula (Fig. 294, A), through a perforation in the ventral valve in 

 Discina (C), and through a foramen in the spout-like posterior end 



