Mr. J. Morris on the genus Siphonotreta. 319 
which separate it from the ordinary Orbicule ; the shell is gene- 
rally more solid and calcareous, both valves are nearly equally 
convex, and the passage for the muscle of attachinent, instead of 
being through a longitudinal fissure as mm Orbicula, is consider- 
ably “contracted, being confined to a small tubular perforation 
situated at the marginal end of a rather deep closed furrow. 
The pedunculated form assumed by the muscle of attachment in 
Orbiculoidea must have allowed greater freedom of motion to the 
animal, and may be the reason for the more conical development 
of the lower valve in this genus, as distinguished from the com- 
pressed form of the same valve in Orbicula. The contracted per- 
foration in Orbiculoidea is well shown in the figure of Orbicula 
Forbesii*, ‘Memoirs Geol. Surv. of Gr. Britain,’ vol. i. pl. 26. 
f. 2, and is alluded to by Mr. Salter in his remarks on this spe- 
cies. This shell appears to be the same as the Schizotreta ellip- 
tica, Kutorga (1847), and is probably the older form of Patella 
implicata, Sow. ‘Sil. Syst.’ t. 12. f. 14a, as well as identical 
with Patella antiquissima, Markl. (His. Let. Suec. t. 12. f. 11, 
and description), and is a type of D’Orbigny’s Orbiculoidea. 
With respect to Obolus, which has not yet been recorded as 
occurring im this country, I have, by the kind permission of 
Prof. E. Forbes, examined the fine collection of Lingule pos- 
sessed by the Museum of Practical Geology, without finding any 
form distinctly referable to Kichwald’s genus. At present this 
shell is peculiar to Russia, being there widely distributed, and 
it appears to be one of the most ancient animal forms with which 
we are acquainted, for the beds containing it are altogether at 
the lowest limits of the fossiliferous deposits of Europe. It is 
somewhat remarkable, as mentioned by M. de Verneuil, that 
notwithstanding the extreme abundance of this shell in Russia, 
it has never been found on the other side of the Baltic, either in 
Sweden or Norway, where however exist grits of similar age to 
those of Russia, below the limestones containing Asaphus eapan- 
sus and Ilenus crassicauda. Nor has it been found in America : 
it appears in that country, as in the British Islands, to be syn- 
chronously represented by the genus Lingula, with which it has 
the nearest affinity ; for Sir C. Lyell mentions that the lowest fos- 
siliferous strata in the United States (those for instance near 
Lake Champlain) contain abundant fragments of Lingula, giving 
to the rock, as in the Obolite gvits of Russia, a‘ very micaceous 
appearance. 
In the Russian specimens of Obolus, I could not detect the 
peculiar reticulated structure of Siphonotreta; the shell is cal- 
* Mr. Gray of Dudley possesses beautiful specimens of this shell, from 
which collection Mr. Davidson described it in the ‘ Bull. de la Soc. Géol. de 
France,’ vol. v. 2nd ser. t. 3. f. 45. 
