.275.J 
59 
Dudley, above the Trenton limestone, but in this country it is charac- 
teristic only of the latter formation, for there it abounds, whilst in the Ro- 
chester shales it is very rare. Occasionally the relative position of two 
formations may be so obscured, or the mineral character so similar, that 
the amount of difference will not be always estimated by those who de- 
scribe them. It is just so in Ohio and Kentucky, where no observer 
has yet drawn the line of distinction between the Trenton and the new- 
er limestone superimposed upon it, because the similarity in mineral 
character and color is so perfect, and the formations thin out into mere 
seams and layers, that a critical knowledge of the fossils is necessary to 
determine the difference in age. It is possible, therefore, the Calymene 
Blumenbachii and Asaphus tuberculatus are most abundant in a lime- 
stone at the base of the Dudley series, which corresponds in age, not 
only with the Trenton limestone, but also with the Llandeile flags of 
Murchison, whose observations, when published more in detail than they 
have yet been given to the world, will probably clear up all this diffi- 
culty. 
The catalogues of organic remains of the Silurian system, hitherto 
published in Europe, are only calculated to mislead and confound the 
geological enquirer, as they are far too vague; not only the species 
peculiar to certain formations being indiscriminately mingled, but even 
the groups of the Silurian and Carboniferous systems are strangely con- 
founded. This is the case in " De la Bache's manual," " Thomson's 
outlines of Geology," Phillips' Treatise on Geology in the Encyclope- 
dia Metropolitana, and in the Lethea Geognostica of Bronn. The last 
named author has published a plate of numerous species of organic 
remains as characteristic of the mountain limestone, among which it is 
difficult to detect one belonging to that system, for they are nearly all 
characteristic of the Silurian strata. There are also importanterrors m 
his figures, such as the round perforation in the beak of Atrypa prisca, 
an imperforate shell. Excepting Lingula, no bivalve with a 'perforated 
apex occurs either in the Silurian or Carboniferous systems. The genus 
Terabratula is wholly unknown, and the shells usually referred to that 
genus I propose to group under the generic name of Stenoscisma, de- 
rived from two Greek words signifying narrow fissure, a character these 
shells possess under the imperforate apex of the larger valve, and which 
serves to connect the genus with Delthyris, from which it differs in 
having no cardinal area. This last named character on the other hand 
connects it with the genus Strygocephalus. I refer to it the common 
Silurian bivalve, Terebratula Schlotheimii, (Von Buch.) The genus 
Producta of Sowerby is unknown in all the rocks of the Silurian sys- 
