24. AUSTRALASIAN ANTARCTIC EXPEDITION. 
In Terebratella the septum joins the cardinalia without bifurcation and the 
hinge trough is enclosed by two flatly inclined and excavate hinge plates. The cardinal 
process is confined to the umbonal region and is transverse. The cavities between the 
hinge plates and the floor of the valve also penetrate to some extent under the floor of 
the hinge sockets. In Pachymagas no such cavities exist, and there are no excavate 
hinge plates. The floor of the hinge sockets is a raised solid platform at the posterior 
end of the valve. The septum bifureates before reaching the cardinalia, the socket 
ridges are swollen and firmly united with the crural bases, and the hinge trough lying 
between them is more or less completely occupied posteriorly by a prominent cardinal 
process. In Gyrothyris there is no cavity below the hinge trough or the base of the 
hinge sockets, both being floored by a solid platform. The septum bifurcates narrowly 
on top just before reaching the cardinalia, but the hinge trough is shallow and obtuse. 
The cardinal process is confined to the umbonal region and is transverse as in Terebratella, 
Gyrothyris resembles Pachymagas more than Terebratella but differs from the 
former in the absence of beak ridges, the shallowness of the hinge trough, and the 
transverse nature of the cardinal process. Further, Gyrothyris mawsoni possesses a 
radial ornament which has never been observed on any species of Pachymagas. The 
genus is probably of similar antiquity, for in the uppermost Mount Brown limestone 
of the Weka Pass district, Canterbury, New Zealand, there is an unnamed species with 
exactly similar shape and beak characters, although larger in size and without radial 
ornament. The age of the rock in which it occurs is Awamoan, probably Upper 
Miocene. 
All the other recent species with Terebratelliform loop from the southern seas 
appear with one exception to be correctly referred to Terebratella, viz., IT’. dorsata 
(genotype), 7’. dorsata submutica Fischer and Oehlert, 7. sanguinea Leach, 7’. sanguinea 
var. Thomson, 7. rubicunda Sow., T. cf. Magella carinata, Thomson, and T. may 
Blochmann. The above list omits the doubtful Terebratella rubiginosa Dall,* concerning 
which very little is known. The exception referred to is Terebratella sp. Jackson (1912) 
from the Burdwood Bank, in which Jackson states that the “ well-developed cardinal 
plateau is fixed to the bottom of the valve, and depressed longitudinally in the form of 
a trough.” The cardinal process is Terebratelliform, the foramen apparently sub- 
mesothyrid with discrete deltidal plates and with well-marked beak ridges. ‘This 
species appears to belong to a new genus. 
Stethothyris approaches Neothyris most nearly in its beak and hinge characters, 
but it appears to belong to an older stock which attained Magellaniform loop characters 
independently. There can be little doubt that the various species of Neothyris have 
developed from species of Pachymagas of the type of P. parki (Hutton). The earliest 
known species of Neothyris, N. novara von Thering, appears in the Awamoan of New 
and not Terebratiform. 
