BRA CHIOP ODA. 
103 
Obolella Sabrince. 
After Davidson. 
Fig. 58. Internal cast of 
brachial valve. 
A crotreta Nicholsoni. 
After Davidson. 
Fig. 59. Interior of brach¬ 
ial valve. 
flanked by cardinal (?) scars, in front of which are two smaller impressions. 
Essentially the same features are seen in Mr. Matthew’s figure* of the interior 
of this valve. The interior of the brachial 
valve of A. gemma is compared by Mr. Wal¬ 
cott to that of the Obolella Sabrince , Calla¬ 
way, as represented by Davidson.-}* The 
agreement is essentially complete in these 
respects, and Mr. Davidson’s figures of the 
pedicle-valve of this species give it a great 
elevation and an indistinct cardinal area, though they do not show the apical 
foramen. It is highly probable that the species will prove an Acrotreta. 
The vertical groove on the cardinal area is, like the elevation and definition 
of the area itself, a very variable feature. In A. subconica , A. gemma , and A. 
Nicholsoni it is sharply defined, while A. disparirugata, Kutorga, A. Baileyi Mat¬ 
thew, and A. socialis, von Seebach, bear less definite indications of this character. 
The suggestion offered by Kutorga, that this furrow may be taken as an “ in¬ 
dication of a deltidium,” may be understood as not implying more than a remote 
homology with the deltidium of the articulate brachiopods. 
We have yet but few American representatives of this genus. In 1860, Mr. 
Billings described the first known American species, A. gemma , and Mr. Wal¬ 
cott (1884) has considered the species regarded by Meek (1872) as A. subconica , 
Kutorga,:}: from the Gallatin River, Montana, and the A. pyxidicula, White (1874), 
from Nevada, as synonymous with it. Acrothele? dichotoma, Walcott (18 4), 
from Nevada, has been subsequently referred by its author (1886) to Acro¬ 
treta, while Acrotreta subsidua, White (1874), is referred to Acrothele. Mr. 
Matthew has described (1885) Acrotreta Baileyi and A. ? Gulielmi, from the St. 
John group; the latter of these is a representative of a distinct genus, which 
is discussed in the following pages under the name Discinopsis. All the 
American species are from primordial faunas; A. socialis , von Seebach, is 
* Illustrations of the Fauna of the St. John Group, No. 3, pi. v, fig. 13. 
f British Silurian Brachiopoda, Supplement, pi. xvi, fig. 27 d. 
I This form Mr. Meek proposed to name A. attenuata, in case it proved distinct from the Russian 
species. 
