BRACHIOPOD A—THOMSON. 
21 
Amphithyris is thus comparable to Argyrotheca in its type of lophophore, but 
has a more primitive type of brachidium, and in addition differs in beak characters, 
Argyrotheca having a hypothyrid foramen. It is comparable in shape and beak characters 
to Platidia, but has a more primitive type of lophophore and brachidium. The only 
other genus in which amphithyrid beak characters are attained is Muhlfeldtia. In 
M. truncata the dorsal umbo is only slightly notched, the dorsal valve is still convex 
and the lophophore is plectolophous. In M. monstruosa and M. echinata the dorsal 
umbo is deeply notched and the dorsal valve is flat or even slightly concave, while 
nothing is known of the lophophore. The loop of Muhlfeldtia is, of course, much more 
advanced. 
Amphithyris must be placed at present in the subfamily Megathyrince, which 
includes those members of the Terebratellidce which have not advanced beyond the 
schizolophous stage of lophophore, together with Megathyris itself in which the lophophore 
is pytcholophous, i.e., with additional lateral lobation. It may be questioned, however, 
whether Platidia is correctly placed in the Dallinince, and should not be regarded as the 
final member of a subfamily which should include Amphithyris and Argyrotheca (in part). 
Platidia differs from all other members of the Dallinince in its loop, lophophore and beak 
characters and in the possession of spicules. Argyrotheca australis (Blochmann) has 
prongs rising from the septum very similar to those displayed by Platidia, and differs 
in its brachidium only in possessing short crura and long and strongly curved primary 
lamellae. The lophophore, which is unknown, may confidently be expected to be 
intermediate between the simple schizolophous type and the type displayed by Platidia % 
Magasella incerta, Davidson, which Fischer and Oehlert refer with doubt to Platidia, 
has hypothyrid beak characters, and its loop resembles that of Platidia but lacks the 
prongs on the septum. If adult, it appears to constitute a new genus in the above 
series. 
The only other species at present referable to Amphithyris is Terebr alula seminulum 
Philippi, which was considered by Davidson (1887) as a synonym of Platidia anomioides, 
but which Fischer and Oehlert (1891) recognise as generically distinct, and perhaps a 
species of Cistella (Argyrotheca ). This species has amphithyrid beak characters, a 
schizolophous lophophore, and apparently no primary loop lamellae. It was placed by 
Davidson in 1852 in his new genus Morrisia, the type of which was left ambiguous. 
Davidson’s first gave a diagnosis and figure of the shell now known as Platidia anomioides 
but gave no name, and then added as belonging to the genus Morrisia seminula Philippi. 
It was, therefore, a case of a genus with two genosyntypes, Platidia anomioides indicated 
by the diagnosis and figure, and Morrisia seminula indicated by name, but not definitely 
stated to be the type. Later in the same year Davidson recognised the ambiguity, and 
definitely selected Platidia anomioides as the type of Morrisia, which thus became an 
absolute synonym of Platidia, founded earlier in the same year. But for this unfortunate 
choice of Davidson’s it would have been possible to employ Morrisia for the new genus 
here described and to avoid the introduction of a new name. 
