90 
Shell ovate, the length and greatest width being nearly equal, or the width greater than 
the length in young or in compressed specimens. The greatest width is placed near the front 
of the shell, the sides curving gently from the beak for about two-thirds of the length, and then 
being bent abruptly inwards. Ventral valve convex, most elevated in the umbonal region, 
curving to the sides, with a broad shallow mesial sinus which is occupied by from three to six 
plications. Beak of the ventral valve small, incurved, and in contact with the umbo of the 
dorsal valve, with a minute circular foramen at its apex. Dorsal valve more convex than the 
ventral, most gibbous a little above the middle, with its central portion elevated into a mesial 
fold, which corresponds with the ventral sinus, and is occupied by from three to six plications. 
Beak buried beneath that of the ventral valve. Surface marked with rounded or sub-angular 
plications, of which those on the mesial fold and sinus bifurcate, whilst all are obszure or 
obsolete towards the beaks. Concentric strize and lines of growth are also present. 
An apparently average specimen has a length of ten lines, and a greatest width of ten 
lines and a half, with a height of six lines. A smaller individual has a length of seven lines, 
a greatest width of seven lines and a half, and a height of three lines and three-quarters. 
According to Hall, the dorsal valve possesses a median septum which reaches more than 
half the length of the shell, and is divided above, so as to leave a triangular pit. There can 
be no question as to the identity of Rhynchonella (?) Laura (Billings), with Hall’s Leiorhyn- 
chus nvulticosta, All the specimens of this species that I have seen, have the greatest width 
slightly in excess of the length, though Hall considers the two diameters to be normally nearly 
equal. The species is readily recognised by its general form, its rounded plications, becoming 
obsolete towards the beak, and not interlocking at the margins of the valves, its mesial fold 
and sinus, and the perforated beak of the ventral valve. 
Locality and Formation.—Hamilton Group, Widder, Township of Bosanquet (Collected 
by Mr. George Jennings Hinde). 
109. LetornyncHus Huronensis (Nicholson) . 
Shell elongate-ovate, the greatest width being from one-sixth to nearly one-fifth less than 
the length, whilst the greatest width is placed nearly about the middle of the shell. Ventral 
valve convex, very gibbous in the umbonal region, abruptly curving to the sides, and depressed 
from about the middle to the front margin of the shell. A very broad and exceedingly 
shallow mesial sinus, which is obsolete near the beak, but occupies almost the whole front 
margin of the shell, and supports about five low- rounded plications. Beak of the ventral 
valve large, prominent, and incurved, apparently not perforated by a foramen, and in close 
contact with the umbo of the dorsal valve. Dorsal valve convex, strongly curved towards the 
sides, its centre occupied by a broad mesial fold, which corresponds with the ventral sinus, 
and carries five rounded plications. Margins of the fold and sinus more or less distinetly 
interlocking. Beak of the dorsal valve buried deeply beneath that of the ventral valve. Sur- 
face marked with rounded plications, which are very obscure, except upon the fold and sinus, 
and which become nearly obsolete in the neighbourhood of the beaks. The plications are 
crossed with very fine concentric striae, and with strong and very well marked lines of growth 
which become close-set and imbricating towards the margin of the valves. 
The dimensions of a large individual are :—length, one inch ; greatest width, ten lines ; 
height, eight lines. A smaller and narrower individual has a length of eleven lines, a greatest 
width of eight lines, and a height of six lines. 
Fig. 28. There is no doubt as to the entire 
distinctness of this from the preced- 
ing, though there may be some ques- 
tion as to its reference to Leiorhyn- 
chus ; since Hall states that the fold 
and sinus in this genus never inter- 
/ lock, which they certainly do in some 
examples of this species. In other 
respects, however, there is a close 
general resemblance between this and 
3 Pesce aaroe Tea Neo a, Dorsal view; b, Ventral L. muilticosta, especially as regards 
view ; c, Side view of a large individual. From the Hamilton Group. , 
? 8 P the surface-characters, and I do not 
