Bracuioropa. | DEVONIAN MOLLUSCA. 38] 
Sp. Ch.—Triangular, pyramidal; receiving valve flattened or slightly convex near the beaks and the 
narrow sides; apical angle about 115°; beak small, projecting with a small triangular foramen beneath it; 
front depressed into a very wide sinus, and abruptly curved towards the entering valve, nearly at right angles 
with the rostral portion (exceeding a right angle in the much elevated varieties, and considerably less in the 
depressed forms) ; lateral margins sigmoid, either entire or with two or three angular notches, from the ends of 
ridges extending a very short way towards the beak; front margin abruptly arched upwards into a sinus, 
varying from triangular or acutely elliptical, and higher than wide, to transversely semielliptical, and the height 
less than half the width, and either without plaits, or with from two to six, varying in the amount of angularity, 
but always becoming obsolete a considerable distance from the beak; entering valve with the profile only 
slightly arched, the greatest depth of all the varieties being at the front margin; rostral portion and sides 
slightly convex, the latter sloping rapidly towards the receiving valve, middle of the front varying from acutely 
angular to broadly arched ; surface under the lens with fine, close, obtuse, concentric striz, varying from six 
to ten in the space of one line, crossed when finely preserved by longitudinal thread-like striae, five or six in the 
space of one line, separated by wider flat spaces. 
There are four varieties which have been separately named, but which pass by the most insensible grada- 
tions one into the other, and certainly not distinguishable by any constant specific character. All the 
varieties are recognised by the marginal plaits disappearing a considerable space in front of the beaks. The 
T. reniformis, which in books is often confounded with this species, is perfectly distinct by the great inflection 
of the receiving valve. The texture of the shell gives a minutely flexuous fibrous appearance to the surface 
under the lens, when partially decorticated, as most specimens are, but infinitely finer than the rarely seen 
longitudinal striation above mentioned. 
Ist Var. acuminata (Mart.)—Front sinus very high, acutely angular, few or no trace of mesial plaits, nor 
lateral marginal plaits, except in very large specimens. Width two inches, proportional length of 
entering valve +, of receiving valve 4, from plane of lateral margin to top of sinus ;;,, depth from 
middle of entering valve at right angles to the surface =. 
2nd Var. platyloba (Sow.)—Transversely ovate, plaits obtuse. Width one inch four lines, length of 
receiving valve ~, from plane of lateral margins to top of sinus =;. 
3rd. Var. pugnus (Mart.)—Rhomboidal tumid, three to six mesial, and three or no lateral, short, strong 
80 
plaits. Width one inch, length 4, from plane of lateral margin to top of sinus =, depth at right 
1009 
angles to middle of entering valve =. This is rather more tumid than the other varieties, and 
includes 7. cordiformis, which M. de Verneuil supposed to be distinct by want of lateral plaits, 
although one of Sowerby’s original figures of that supposed species shews them. 
4th Var. mesogona (Phill.= A. triangularis Sow.)—Form and other characters exactly as in the type 
var. acuminata, but the width less than one inch. This cannot be the young of the large acute forms 
(which are depressed), but may be a dwarf variety. 
Position and Locality—Vars. 1, 2, and 3, not uncommon in the Devonian limestone of Plymouth, S. 
Devon; var. 4, rare in the slate of Petherwin, Launceston, Cornwall (the original specimen of A. triangu- 
laris has the apex of the sinus bifid, and traces of two minute lateral plaits, as in Phillips’s description of 
T. mesogona). 
HemitHyris cuBoIDES (Sow. Sp.) 
Ref.= Atrypa cuboides Sow. Geol. Trans. 2nd Series, Vol. V. t. 56. f. 24. 
Sp. Ch.—Globose, subcuboidal ; entering valve moderately convex in the middle, sides slightly convex, but 
abruptly sloping nearly at right angles with the middle, and subparallel to each other; lateral margins nearly 
horizontal, very slightly sigmoid ; front margin abruptly elevated into a very large parallel-sided square sinus, 
slightly rounded at its end, which only produces a faint indication of a wide mesial ridge at the margin of wide 
specimens ; entering valve nearly flat, with a small, pointed, incurved beak, beneath which is a very small 
triangular opening ; mesial sinus only marked at the point where the lateral margins turn up, the large square 
extension of the front of the valve forming a slightly convex or very slightly concave filling to the sinus ; 
