BBACH10P0DA—THOMSON. 
23 
The species differs from Amphithyris seminula (Philippi) in the presence of the 
radial ornament in the ventral valve. That species has moreover a smaller dorsal 
foramen and a smaller and less invaginated lophophore according to Philippi’s figures 
as reproduced by Davidson. 
Subfamily Magellaninas Beecher. 
The subfamily Magellanince includes a number of genera with loops of a pre- 
Magadiniform, a Magadiniform, a Magelliform, a Terebratelliform, or a Magellaniform 
pattern, or of a pattern derived from one of these, each genus attaining its final loop 
form by a series of metamorphoses corresponding to all of the above patterns which 
precede it in the order above stated. Thus the genus Terebratella passes through pre- 
Magadiniform, Magadiniform, and Magelliform loop stages during development. There 
are, however, a number of different stocks which have by similar but independent series 
of loop metamorphoses attained Terebratelliform or Magellaniform loop characters, and 
it has been found that some of these stocks can be satisfactorily distinguished by 
characters of the beak and cardinalia. A study of the fossil forms of New Zealand and 
Australia with regard to these latter characters indicates that there are other forms 
with Terebratelliform or Magellaniform loops which cannot be placed in any of the 
known genera, and the same is probably true of some of the recent species that have 
been placed under Terebratella and Magellania. Comparisons of the fossil or of the 
existing faunas of the various southern lands and seas cannot possess much weight 
until these different stocks have been distinguished. 
In the present collection there are two species for which new genera become 
necessary. These I propose as follows : — 
Gyrothyris, genotype Gyrothyris mawsoni sp. nov., Macquarie Island, loop 
terebratelliform. 
Stethothyris, genotype Stethothyris uttleyi sp. nov., Ototaran, New Zealand, 
loop magellaniform. 
Gyrothyris stands in most of its characters between Pachymagas and Terebratella, 
but in beak characters it differs from both. These two last genera have a mesothyrid 
foramen with fairly well-marked beak ridges, whereas in Gyrothyris beak ridges are 
practically absent. There is a difference between the hinge teeth of Terebratella, and 
Magellania on the one hand and Pachymagas and Neothyris on the other, to which 
attention has not previously been called. In the former pair the hinge teeth are rather 
slender and spring from the overhanging dorsal sides of the posterior part of the ventral 
valve. Their bases are not greatly swollen and do not interrupt the internal curvature 
of the valve at the points of origin, the inner and outer surfaces being approximately 
parallel. In Pachymagas and Neothyris the hinge teeth are larger and spring from 
swollen bases which depart greatly from parallelism to the outer surface and con¬ 
siderably restrict the beak cavity. In Gyrothyris the teeth resemble those of Pachymagas 
in these respects. 
