Bracuioropa. | UPPER PALAZOZOIC MOLLUSCA. 425 
smaller than the lateral ones. Width one inch four lines, proportional length of receiving valve varying 
from 3% to %, length of entering valve = in the former, * in the latter, depth of both valves about & in the 
former, & to & in the latter. 
The most remarkable character of this variety, besides its gibbosity, is the abrupt raising of the mesial 
sinus at the margin, nearly at right angles to the plane of the lateral edges, shortening the length of the middle 
of the ridge, notching the front margin, and giving a nearly semicircular curve to the profile of the receiving 
valve, arched as abruptly towards the front edge as towards the beak, also the less transverse elongation, narrow 
cardinal area, and approximate beaks, as well as perhaps one or two more ribs than ordinary in the sinus of 
old specimens; but in all these characters it varies, so that the gradual passage into the S. trigonalis is certain. 
Thus elongate specimens, with subparallel sides, short hinge-lines giving nearly right angles with the sides, have, 
in some cases, only the ordinary three or four thick ribs on the mesial ridge, and have the lobe of the receiving 
valve filling the sinus in the margin, projecting almost as much, and forming as acute an angle with the plane of 
the lateral edges, as in the ordinary S. trigonalis, in which latter also the beaks may occasionally be seen as 
approximate as in the S, disuleata. On the other hand, all the characters of the typical varieties of S. disud- 
cata may be seen in specimens having the transverse elongation of S. trigonalis ; so that although a very marked 
variety, I have no hesitation in saying that, as a species, it should be united with Martin’s type. Some of the 
specimens shew the muscular impressions rather larger and more deeply marked than in the var. ¢rigonalis ; but 
this is a common character of the deep varieties of all shells. Outside the adductor impressions in the receiving 
valve (which are separated by a slender mesial septum not reaching the beak) is a coarsely punctured space, 
from which the very slender thread-like ovarian impressions extend towards the front. The sulci bounding the 
mesial ridge are strong, but not so remarkably so as to form a specific character, as Mr Sowerby thought. One 
Derbyshire specimen in the collection, agreeing perfectly with the ordinary types in other respects, is remarkable 
for having a short triangular cardinal area, as in S. crassa of de Koninck. 
Position and Locality —Common in the grey limestone of Derbyshire; rare in the dark limestone of Isle 
of Man; not uncommon in the dark carboniferous limestone of Dallmellington, Dumfriesshire ; rare in the 
limestone of Lowick, Northumberland. 
Var. y. SEMICIRCULARIS (Phill.) 
Ref.—Phill. Geol. York, t. 9. f. 15, 16. 
Desc.—Receiving valve nearly semicircular, twice as wide as long; cardinal area generally very wide, 
concave, sides subparallel, beaks widely separated ; mesial ridge angular or depressed, with usually about six or 
eight ridges at the margin; each side with fourteen to seventeen strong, subangular ribs, which either remain 
simple to the edge, or a few of them divide imperfectly into two or three by shallower sulci than divide the 
primary ones, (often imperfectly divided on one side of the shell and simple on the other). Width one inch nine 
55 16 
lines, proportional length of receiving valve =;, of entering valve =, width of area ;% to ;4, depth of both valves 
varying from = to 3. 
This shell, which at first sight seems so distinct from S. ¢rigonalis, passes into it by the diminution of the 
cardinal area and all other characters. The specimens in which most of the ribs imperfectly di- or tri-chotomise 
look remarkably different from the original types of S. trigonalis ; but the specimens having the typical characters 
in all other respects of the S. semicircwlaris, vary to a singular degree in this respect, some specimens having all 
the lateral ribs perfectly simple, others having a variable number subdivided ; and still more interesting are the 
not uncommon specimens having simple ribs on one side and duplex ones on the other. 
Position and Locality—Common in the carboniferous limestone of Isle of Man. 
