BRACHIOPODA. 



103 



OhoUllti Sohrince. 



After Davidson. 



Fig. 58. Internal cast of 



brachial lalve. 



Acrotreta Nicholsoni. 

 After Davidson. 

 Fig. 59. Interior of bracli 

 ial valve. 



flanked by cardinal (?) scars, in front of whicli are two smaller impressions. 

 Essentially the same features are seen in Mr. Matthew's figure* ol' 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 Sabrimc, 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.pijxidicula, 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, Wliite (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, tig. 13. 

 t British Silurian Braehiopoda, Supplement, pi. xvi, Hg. 27 d. 



t This form Mr. Mekk projiosed to name A. attenuata, in case it proved distinct from tlu- Russian 

 species. 



