20 AUSTRALASIAN ANTARCTIC EXPEDITION. 
Family TeREBRATELLIDA King. 
Subfamily Mecatuyrina: Dall. 
AMPHITHYRIS genus nov.. 
Genotype AMPHITHYRIS BUCKMANI sp. nov. 
Shell plano-convex, with beak characters similar to those of Platzdia, viz., ventral 
valve with sharp beak ridges meeting in an apex dorsally of which there is an open 
triangular delthyrium; dorsal valve with a semi-circular foramen replacing the umbo. 
The pedicle opening consists of a triangular delthyrium behind and a semicircular 
foramen in front, and the very short pedicle issues perpendicularly to the flat dorsal 
valve, enabling the shell to lie close against its swpport with the convex limpet-like 
ventral valve uppermost. Buckman (1916) has shown that the foramen in terebratuloids 
may shift ventralwards of the beak ridges from the hypothyrid to the epithyrid 
position. In this and other similar cases the foramen has moved dorsally, resorbing 
the dorsal umbo. The name of the genus is intended to indicate the position of the 
foramen in respect of the valves, and this type may be called amphithyrid. 
The genus is distinguished from Platidia by its more primitive lophophore and 
brachial support. The lophophore is in the stage termed by Beecher (1897) the schizo- 
lophous, with inwardly directed cirri, 7.e., the lophophore is relatively large and reniform, 
with a simple invagination in front dividing it into two lobes as in Argyrotheca. The 
lophophore of Platidia is much more complex, consisting of a small median posterior 2 
lobe or disc around the mouth with the cirri directed ventrally and outwards, and two 
larger symmetrical anterior lobes with the cirri directed dorsally and outwards. The 
lophophore in passing from the posterior into the two anterior lobes becomes twisted = 
on itself. For a more detailed account the description of Fischer and Oehlert (1891)¢ 
should be consulted. Beecher has cited Platidia as possessing the zugolophous type* 
of lophophore, which is an intermediate stage between the schizolophous and the plecto- 
lophous, the latter being the final stage in the Terebratulacea with a well-developed 
median dorso-ventrally rolled spiral arm. The lophophore of Platidia, however, departs 
widely from the zugolophous stage as represented in young examples of the Dallininaw 
in the so-called Platidiform loop stage, and represents a special type of its own. Fischer 
and Oehlert (1891) have pointed out that a dorso-ventral rolling of a spiral arm is 
precluded in this genus by the restriction of the visceral cavity imposed by the flattening 
of the dorsal valve, and that the lophophore has had to develop in the plane of the 
commissures. 
Amphithyris differs further both from Argyrotheca and Platidia in its brachial] 
support, which consists solely of a centrally situated, fairly high, median septum, with 
no trace of descending or ascending branches of a loop. For the rest the lophophore is 
supported by spicules. In Argyrotheca there is a calcareous band roughly parallel to 
the ‘sides of the shell, corresponding to the primary loop of higher forms. In Platidia 
the septum is placed more posteriorly, and has in consequence a shorter gpanlery, loop, 
and in addition a pair of prongs at the top of the septum, ; 
