BRACH10P0DA. 
135 
development as a conspicuous character of the interior; vide D. reversa ? PI. 96, 
fig. I, 3 a, 4 a, 5 a, 6 a ; D. tarda , PI. 96, fig. V, 4 a ; D. rugata ?, PI. 98, fig. I, 
6 c; D. intermedia, PI. 99, fig. YI, 1 a, 2 a, 3 a, 4 a (Systeme Silurien, vol. v, 
1879). 
The fossils which have been referred to Orbiculoidea, Davidson, not 
D’Orbigny, the best known of which are 0. Forbesi, Davidson, and 0. conica, 
Dwight, differ from all the foregoing in having the perforated valve very con¬ 
vex and the imperforate one depressed-conical (0. Forbesi), or flat (0. conica ). 
This convexity of the pedicle-valve attains an amazing development in Pro¬ 
fessor Dwight’s remarkable species, the umbo being prolonged into a high cone, 
with the apex inclined toward the anterior margin of the shell. The same 
inclination is observable in specimens of 0. tenuilamellata, from the Niagara 
group at Hamilton, Ontario, but only to a slight degree. 
The pedicle-groove has essentially the character seen in Orbiculoidea, 
D’Orbigny, but is usually much more distinctly retained on account of the 
greater thickness of the shell, which is in striking contrast to its tenuity in the 
last named group. A question may fairly arise as to the advisability of con¬ 
sidering the differences mentioned in these two groups as of generic importance, 
but it was upon a strictly congeneric species that Kutorga established the genus 
Schizotreta, a name which has uniformly been regarded as a synonym for 
Discina — Orbiculoidea, Davidson, since Professor Morris’ observations on the 
subject in the Annals and Magazine of Natural History in 1849.* 
To apprehend the proper status of this group, Kutorga’ s diagnosis is ap¬ 
pended : 
Schizotreta, Kutorga. Ueber die Siphonotretaeae; Yerhandl. der russ.-kais. 
mineral. Gesellsch. zu St. Petersburg, pp. 272, 273. 1848. 
“ The chief character of this genus lies in the peculiar external slit-shaped 
pedicle-aperture, the position of which is precisely the reverse of that in the 
preceding genera [Siphonotreta, etc.]. The apex of the beak is greatly ele¬ 
vated, but far from being in the center of the valve, and from it passes over 
the strongly convex cardinal slope of the cone, toward the posterior margin, a 
* Second Series, vol. iv, p. 315. 
