Mr. J. Morris on the genus Siphonotreta. 317 
chief character consists in a short, perfectly straight, perforated 
beak, never bent towards the ventral valve. The walls of this 
beak are very thick, and hence it does not appear, as for instance 
in the Terebratule, hollow within, but solid and perforated by a 
narrow sipho, which serves for the reception of a cylindrical 
muscle of attachment. 
“The beak presents two chief diversities of form: it is either 
drawn away, in very different degrees, from the hinge-margin 
towards the centre of the dorsal valve; that is, is placed at a 
greater or less elevation above the hinge-margin,—or it lies ex- 
actly in the same plane with the hinge-side of the dorsal valve. 
In the first case the dorsal valve has properly the form of a cone 
more or less inclined towards the hinge-side, and the sipho ap- 
pears either as a perfect tube (Siphonotreta, Acrotreta), or as a 
tube opened up externally for a portion of its length from the 
apex of the cone (Schizotreta). In the second case the dorsal 
valve represents only the half of a cone, in which the shorter 
hinge or posterior part has been cut away from the apex to the 
basis, exactly in such a manner that the external opening of the 
beak is changed into a groove less than a semicircle in depth, 
and the sipho into a semicylindrical groove open along the whole 
length of the hinge-surface (Aulonotreta). See Pl. VII. 
“In no portion of the shell of this group can we observe the 
slightest indication of a predominance of development ; the cen- 
tral part is not distinguished from the marginal portions ; hence 
neither valve shows either a carina or a sinus ; the hinge-sides 
form together an arch, and pass imperceptibly into the lateral 
margins; there are no wing-shaped expansions of the hinge- 
margins, and finally, neither the cardinal nor anterior margins 
exhibit either folds, serratures, or excision. 
“The anatomical structure of the shell of the Siphonotreteze 
is this. The whole inner surface is covered by a continuous layer 
which is so thin that it welds itself closely to, and takes the form 
of, all the larger prominences and folds of growth. This layer, 
from its position and colour, I shall call the nacreous-layer (Perl- 
mutterschicht). The external surface of the shell is also covered 
by a continuous, but considerably thicker, corneous epidermal- 
layer, which is so much developed, and from its horny texture 
has so great durability, that sometimes, even when all the other 
layers are dissolved and vanished, it is still perfectly preserved— 
a peculiarity which, in the whole family of Brachiopods, is found 
only in this group and in the Lingule. This epidermal-layer also 
covers the inner wall of the sipho in all its diversity of forms. 
Lastly, the part between these two layers, and always the thickest, 
is the proper calcareous shell.” 
Any remarks upon the above characters must be considered 
