Dr. J. Allan Thomson—On the Terebratellide. 499 
growth of the primary lamelle in the Dallinine commences both 
from the septum and the crura, as is the case in the Magellanine, or 
from the crura only. In the Magellanine the completion of the 
primary lamelle is attained only after the hood has developed into 
a ring, and on its completion the Magadiniform stage is reached. In 
this stage the primary lamelle and the ends of the ring are separately 
attached to the septum, and at a considerable vertical distance apart. 
In Platidia, as figured by Fischer & Oehlert (1891), the same is 
true, but in the earliest known Platidiform stages of Macandrevia and 
Dallina the attachment of the primary lamelle is very oblique, and 
the anterior part unites with the end of the hood. A similar oblique 
attachment and union is not attained in Zerebratella till a later stage, 
the Magelliform, when the ring has attained a considerable size and 
is widely open. ‘ 
Some similarity exists between late Platidiform and Ismeniform 
stages of Macandrevia and Daliina on the one hand and early Magelli- 
form stages of Zerebratella on the other. In all of these the primary 
lamelle are complete, and run forward obliquely up the septum to 
unite with a ring above. The differences are that in the Platidiform 
and Ismeniform stages the ring is not so large nor so widely open, 
while its lower ends project forwards into two divergent points, 
whereas in MMagella and in Magelliform stages the lower ends are 
more or less rounded. ; 
The chief difference in later stages is that in the Dallinine lacune 
open on the lower sides of the ring and so produce a Frenuliniform 
stage which has no counterpart in the ontogeny of Zerebratella or 
. Magellania. 
MGHLFELDTIA AND ITs ALLIES. 
Mihifeldtia truncata differs in its ontogeny from members of 
Dallining in that the secondary part of the loop appears as a ring 
before the primary lamelle appear. Furthermore, as in the Magel- 
lanine the primary lamelle grow from each end to unite in the middle. 
There appear, however, to be other features in which differences from 
the Magellanine exist, and some resemblance to Dalliniform ontogeny 
may be traced. The high board-like septum of the early stages of 
the Magellanine does not appear to exist so far as one may judge from 
the figures of Deslongchamps (1884) and Fischer & Oehlert (1891). 
Certainly in Kraussina and Megerlina the septum is quite low. The 
ring above the septum in the earliest; known stages of Miihifeldtia is 
different in shape and position from the ring in pre-Magadiniform 
and Magadiniform stages of Terebratella. It lies further forward on 
the septum, and the lower sides exhibit forward extensions not 
shown in the early stages of Terebratella. At alater stagein I. trun- 
cata, and in the adult brachidium of MMegerlina Lamarckiana, small 
points which represent the anterior beginnings of the primary 
lamelle appear, not on the septum, but on the lower outer sides of 
the ring, if Deslongchamps’ figures may be trusted. This difference 
both from the Dallinine and the Magellanine appears sufficiently 
fundamental to necessitate the recognition of a subfamily to include 
Mihlfeldtia, Megerlina, and Kraussina. Before such a step is taken, 
