410 BRITISH PALASOZOIC FOSSILS. [Bracniopopa. 
the adult; greatest width a little in front of the middle ; receiving valve moderately gibbous, profile almost 
regularly arched, greatest depth at rather more than one-third from the beak; beak large, obtusely angulated on 
the sides from a longitudinal concave impression on each side near the edge; sides moderately convex, sloping 
rapidly to the margins ; mesial portion flattened, becoming gradually concave towards the front margin, in old 
specimens (after one inch three lines long) ; foramen large, approaching close to the beak of entering valve. 
Entering valve similar in shape to the receiving one, but tapering more rapidly towards the beak, evenly and 
moderately convex ; profile regularly arched, depth greatest in the middle, in small ovate specimens (half an 
inch long), after which the lateral portions slope rapidly to the margins, and the mesial somewhat elevated third 
flattened when the front edge is straight, or forming a wide mesial hollow, resembling in size and shape that of 
the receiving valve, when the front margin is concave; from the equal mesial hollow in both valves the front 
margin is not elevated, but remains in the plane of the lateral edges; surface marked with close, quincuncially- 
arranged granular perforations under the lens, sometimes shewing, when partially decorticated, a few thread-like 
longitudinal strize near the front of the lateral margins. Length of adult one inch seven lines, proportional 
length of entering valve +, width 7, width of front =, depth of receiving valve ,j;, depth of entering valve =. 
Younger ovate specimens one inch three lines long have the proportional width 7, depth of both valves = ; 
front undefined. 
This species is remarkable for the nearly equal flattening or concavity of the mesial portion in both valves, 
so that the front edge is not in the least raised. It is quite possible that this may be Axomites attenuatus of 
Martin, as suggested by M. de Verneuil; but as there is no figure given, and the description is not at all 
sufficient for recognising the species with certainty, it is but just to Mr Sowerby to use his name, as well as 
being the more certain course, and in accordance with common usage. It is wider and larger than the 
iS. sacculus Mart. Sp., more depressed, and with much more acute margins, besides wanting the mesial hollow 
until after an inch from the beak, at considerably less size than which the greatly thickened margins shew the 
S. sacculus to be adult; the young of S. hastata being ovate, depressed, and with a thin rounded front, bears no 
resemblance to the S. sacculus, from wanting the notch in the front, and narrow mesial sulci. The internal ap- 
pendages which I formerly figured (in my Synopsis of the Irish Carboniferous Fossils) were not complete, but 
should have had the recurved portion completing a loop, as in the generic character, as first mentioned to me 
by Mr Morris, from the observation, I believe, of Mr Davidson. 
Position and Locality —Common in the carboniferous limestone of the Isle of Man, 
SEMINULA JUVENIS (Sow. Sp.) 
Syn. and Ref. = Atrypa juvenis Sow. Geol. Trans. 2nd Series, Vol. V. t. 56. f. 8. ? Zerebratula id. Phill. 
Pal. Foss. t. 35. f. 165. 
Desc.—Longitudinally obovate, the greatest width nearer to the beak than to the front margin ; posterior end 
broadly rounded, the long, anterior, lateral margins converging, so that in most specimens the narrow end of the 
ovate outline is towards the front; receiving valve depressed, gently and very evenly convex along the middle, 
without trace of mesial flattening or furrow ; profile regularly arched from the front margin to within one-third 
the length from the beak, from which to the apex the curve is much more rapid; beak broad, much incurved, 
small, very slightly projecting, truncated by the moderately large foramen, which reaches very close to the 
margin; sides from the beak along the anterior lateral margins as far as half the length of the shell, with a 
narrow flattened space, defined from the broad slightly compressed beak, by an obtusely rounded nearly rect- 
angular ridge; lateral margins with a broad, nearly uniform curve from the beak to the front margin, the 
convexity towards the receiving valve; front margin narrow, straight, or with a very faint wave towards the 
receiving valve; dental lamellz nearly parallel, short, widely separated; trace of mesial septum in receiving 
valve at about half the length of the shell; entering valve flattened; profile nearly straight in the mesial 
portion, slightly sloping from the beak to the front margin; lateral portions sloping rapidly to the anterior 
lateral margins, but sloping very slightly on the posterior half till close to the edge, where they are obtusely 
deflected to meet the similar flattening of the sides of the other valve; surface smooth to the naked eye, or 
