Mr. J. Morris on the genus Siphonotreta. 319 



which separate it from the ordinary Orbicula ; the shell is gene- 

 rally more solid and calcareous, both valves are nearly equally 

 convex, and the passage for the muscle of attachment, instead of 

 being through a longitudinal fissure as in 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. ii. pi. 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 Schisotreta ellip- 

 tica, Kutorga (1847), and is probably the older form of Patella 

 implicata, Sow. c Sil. Syst/ t. 12. f. 14 a, 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 in this country, I have, by the kind permission of 

 Prof. E. Forbes, examined the fine collection of Lingulce pos- 

 sessed by the Museum of Practical Geology, without finding any 

 form distinctly referable to Eichwald'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 expan- 

 sus and lllcsnus 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 grits 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. Geol. de 

 France,' vol. v. 2nd ser. t. 3. f. 45. 



