498 Dr. J. Allan Thomson—On the Terebratellide. 
preferable term, since Beecher had really Prenulina sanguinolenta' in 
mind when he spoke of WMiihlfeldtia, for although he mentioned 
IM, truncata he used in his illustrations I. sanguinea = Frenulina 
sanguinolenta. 
The so-called ‘ Platidiform’ stage of the loop in the Dallinine is 
not strictly comparable to the brachidium of Platidia, as is shown 
below, and does not appear to be represented in the adult brachidium 
of any known genus, but it can hardly be doubted that a genus with 
such characters will one day be found. 
Jackson’s reasons for removing DMiuhlfeldtia truneata from the 
Dallinine and placing it in the Magellanine are the absence of dental 
plates, the resemblance of one of its early loop stages to an early loop 
stage of Terebratella dorsata, and the appearance of the secondary 
loop before the appearance of the primary lamelle. Deslongchamps 
showed clearly in 1884 that the young loop stages of Megerhia 
truncata (= Mihifeldtva truncata) form a close parallel with the adult 
brachidia of Kraussina and Megerlina, and it is somewhat remarkable 
that Beecher overlooked this resemblance and did not suspect the 
generic distinctness of ‘‘ Miihifeldtia truncata” and ‘‘ Mihlfeldtia 
sanguinea” 
DIFFERENCES BETWEEN DatiiniFoRM AND MaAGELLANIFORM OnroGENY. 
Before discussing whether Dihlfeldtia may be admitted into the 
Magellanine, it is desirable to analyse the difference in loop 
development between that family and the Dallinine. Beecher 
pointed out that in the lower genera the median septum is generally 
low in the Dallinine and projecting above the loop in the 
Magellanine. In Bouchardia, Magas, and Magadina it almost touches 
the opposite valve. In the young growth stages of the higher genera 
this difference between the subfamilies is not so marked, for the 
early platidiform stages of Macandrevia show a high septum. It 
remains true, however, that a high septum persists longer in the 
Magellanine than in the Dallinine. There is also aslight difference 
in the form of the septum, which is more elongate and board-like in 
the Magellanine. 
In both subfamilies the secondary part of the loop appears first as 
a small hood? on the septum, with the opening upwards and forwards. 
In the Magellanine this hood is confined to the posterior, slightly 
lower, end of the septum, but in the Dallinine it projects further 
forwards. At this stage there is an important difference, emphasized 
by Jackson, viz., that in the Dallinine the primary loop is complete 
from the crural bases to the septum, whereas in the Magellanine it 
is imperfect. It does not appear to be yet known whether the 
1 Anomia sanguinea, Chemnitz, being polynomial, Gmelin’s name must be 
used for this species as Dall has suggested. Beecher’s illustrations are based 
on those of Deslongchamps (1884), who referred to it as Terebratella sanguinea. 
It is, of course, a different species from Terebratella sanguinea, Leach, which 
was known. at that date as Terebratella cruenta. 
2 The earliest stage of Terebratella dorsata described by Fischer and Oehlert 
(1892), which shows the secondary loop, has a ring on the septum, but I have 
detected an earlier stage with a hood in 7’. rwbicunda (Thomson, 1915, No. 3). 
