367 
of the Ventriculidse of the Chalk. 
This form will readily be distinguished from every other not only 
by the difference in the primary fold from that of any species which 
approach it in the character of the brachial fold, but in the essential 
character of that brachial fold itself. The narrowness, in com- 
parison with the length, and the close setting of the tubes, are 
together found in no other 
species. The accompany- 
ing figure, which is a trans- 
verse section of a specimen 
of this species, near the top, 
and just therefore missing 
the central cavity, could 
be presented by a similar 
section of no other species. 
It seems to me that there 
is generally no marked di- 
stinct opening to the cen- 
tral cavity, but that it is 
surrounded on all sides by 
the tubes. In fig. 7 of 
PI. XV. the upper part was 
cut away before it came into my possession, but the wall of the 
cavity is, in that specimen, rounding inwards, and not expanding. 
It is important to notice that each brachial fold is distinctly 
tubular and prominently projecting, and that its termination is 
slightly contracted, in the present species, two characters in which 
it essentially differs from B. labrosus fenestratus^ and cha- 
racters which must obviously have materially affected the access 
and circulation of the sea-water. The central cavity into which 
those tubes open is another character to which the last remark 
strongly applies, and is a character also at once distinguishing 
this species from B. fenestratus. 
This species is from the Middle Chalk. 
5. Brachiolites fenestratus. PL XVI. fig. 3. 
Membrane simple and without any primary fold : braehial fold 
in narrow tubes anastomosing and opening into eaeh other in 
not very regular figure, but in several vertical and horizontal 
planes, leaving interspaces between them about equal to their 
own width : each tube rounding at mouth and projecting 
slightly beyond the plane of the most external range of the 
anastomosed mass. 
The description will at once show wherein this species differs 
from the last. A single fragment of tube may be mistaken, as' 
the primary fold is not strikingly marked in either, and the size 
Fig. Q. 
