BRACHIOPODA—THOMSON. 25 
far as present knowledge goes, to the Ototaran, which is a still earlier stage. The ventral 
valve of Stethothyris uttleyi shows no great difference from that of Neothyris, the foramen 
being rather small, mesothyrid, attrite, with fairly well marked beak ridges. ‘The 
hinge teeth are rather small and the bases not greatly swollen, except in old shells, but 
the beak cavity is considerably restricted by a general posterior thickening of the shell. 
The muscular impressions are well marked, as in Neothyris, with a raised median ridge 
between the adductor impressions. 
In the dorsal valve (Plate XV, fig. 26) the septum bifurcates narrowly before 
joining the cardinalia. There is a solid raised platform at the posterior end of the shell 
above which the cardinalia rise and below the surface of which the hinge sockets are 
slightly countersunk. There are no excavate hinge plates. The crural bases are 
considerably swollen, lying within the diverging socket ridges, and they nearly unite 
posteriorly, leaving between them a hinge trough which is long, narrow and shallow. 
The cardinal process consists of three parts, a slightly raised central boss similar to that 
of primitive Pachymagas (cf. Thomson, 1915, No. 1, fig. 2a) and two lateral swellings 
which project on each side of the umbo and at first sight seem to be backward con- 
tinuations of the socket ridges. 
The principal differences between the cardinalia of Stethothyris and Neothyris 
may be summed up by stating that in the former the hinge trough is shallower and 
} g g 
longer and the cardinal process less advanced, not filling the hinge trough. 
In the only example of Stethothyris pectoralis (Tate) in which I have been able 
to expose the cardinalia, there is a similar arrangement of the parts (Plate XVII, fig. 
60), but the hinge trough is almost non-existent apparently owing to abnormal growth. 
Neither the crural bases nor the lateral parts of the cardinal process are so swollen. 
In Stethothyris antarctica sp. nov. the cardinalia almost exactly reproduce those 
of S. pectoralis, with the difference that the hinge trough is well defined and the cardinal 
process a little less advanced, being transverse and little different from that of 
Terebratella. 
By the recognition of this genus it now becomes possible to obtain a clearer idea 
of the history of the forms previously placed under Magellania. Rhizothyris, so far as 
is at present known, is confined to the Oamaruian of New Zealand, [R. rhizoida (Hutt.), 
R. curiosa Thomson] and the older Tertiary of Australia [R. corioensis (McCoy)], and 
did not survive into the Wanganuian of New Zealand, although possibly, as is suggested 
below, it may have survived to a later date in other areas. Stethothyris occurs in the 
Ototaran (middle Oamaruian) of New Zealand (S. uttleyi Thomson) and the older 
Tertiary of Australia [S. pectoralis (Tate)], and still survives in the Antarctic seas 
(S. antarctica). Neothyris first appeared in the Awamoan (Upper Oamaruian) of New 
Zealand [|N. novara (von Thering)], was abundant in the Wanganuian of that country 
[N. ovalis (Hutton), N. campbellica (Filhol) and other unnamed species], and still lives 
in the New Zealand seas (N. ovalis, N lenticularis (Desh)]. Magellania is not found . 
*20218—D Vor. IV, Parr 3, 
