SUB-ORDER E CYRTOCHOANITES 531 
Streptoceras, Bill. Like the last but more arcuate, with laterally contracted 
aperture, and a short hyponomie sinus distinct from the brachial area. Silurian. 
Family 9. Trimeroceratidae. 
Smooth breviconic orthoceracones and cyrtoceracones similar to Poterioceras in aspect 
and sutures, but more slender, especially in the young, and aperture very distinet im 
primitive forms. Even the latter usually have brachial distinetly marked off from 
hyponomic area by ingrowth of sides of the aperture, and in all specialised shells the 
hyponomie sinus and special inflections known as “ brachial sinuses” are formed by bases 
of the arms on edges of brachial area, Finally, the aperture becomes reduced to w more 
or less Y- or T-shaped figure, with an open semicircular sinus at the end of the hyponomac 
shit or area, and similar sinuses in the edges of the brachial slit, corresponding to the 
number of arms. All Silurian. 
Mandaloceras, Hyatt (Dimorion, Barr.), (Fig. 1084). Differs from Potertoceras in 
the gerontic aperture, which is laterally contracted, and has hyponomic and brachial 
areas distinctly differentiated in all but the most primitive 
species. More specialised forms have these areas narrowed 
down, but special sinuses are not formed, 

Fic. 1085. 
Ries eae 2 Tetrameroceras Panderi, Barr. 
Mandaloceras (Gomph.) Bohemicum, Barr. sp. Silurian (Ktage E); sp. Silurian (E); Dvoretz, 
Dvoretz, Bohemia. A, Side view of conch. JL, Aperture. Bohemia. 1/; (after Barrande). 
Trimeroceras, Hyatt (Trimorion, Trimeres, Barr.) ; Pentameroceras ; Septameroceras, 
Hyatt. Silurian. Aperture in the first has a median and two brachial; in the 
second a median and four brachial; and in the last a median and six brachial 
sinuses. 
Hemiphragmoceras, gen. noy. Compressed endogastric cyrtoceracones having a 
narrowed hyponomic area like Phragmoceras, but with brachial areas as in Dimeroceras. 
Type H. (Phrag.) pusillum, Barr. sp. 
Tetrameroceras, Hyatt (Tetramorion, Tetrameres, Barr.), (Fig. 1085). Like the last, 
but with more highly contracted aperture and four lateral sinuses. 
Hexameroceras, Hyatt. Brachial area with six lateral smuses. Octwmeroceras, gen. 
noy. Brachial area with eight lateral sinuses. Type 0. (Phragm.) callistoma, Barr. sp. 
Family 10, Phragmoceratidae. 
Smooth breviconic cyrtoceracones and gyroceracones rapidly expanding by growth in 
their dorso-ventral diameters, and having open apertures only in primitive types or the 
young and ephebic stages of more specialised forms. In the latter gerontic apertures are 
laterally contracted and have a very long hyponomic area terminated by a large 
hyponomic sinus. The brachial area may be more or less open and elliptical, or 
