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[Assembly 
tem, except in the highest or the Ludlow strata, where two species 
occur; here on the contrary, the genus Strophomena of Rafinisque is un- 
known, as well as in the Carboniferous system, whilst it abounds in the 
Helderberg and Trenton limestones. The following table will show 
some of the characteristic genera of the two systems and subdivisions. 
Catenipora, (medial strata.) 
Among the curious and interesting organic remains of the Transition, 
the fucoids are not the least worthy of notice, and they seem to have 
been much neglected by naturalists. They are even more characteristic 
than the testacea, so far as the species have been determined, for parti- 
cular species are more absolutely limited to the respective rocks in 
which they originated. They assume a variety of forms, no doubt cor- 
responding to various as yet undetermined genera. Some of them have 
been of a fibrous reticulated structure, having vescicular appendages, 
often lobed and imitating on the sandstones the forms of tracks of rep- 
tiles and birds which some writers have believed them to be. On plate 
26, in Buckland's Bridgewater treatise, these foot shaped vesicular 
fucoids may be seen attached to the net -work, and no doubt they per- 
formed the office of floats to support the fibrous structure to which they 
were appended. Some naturalists have doubted the vegetable origin of 
these singular remains, but they could neither have been polyps nor 
radiated animals, since no trace of organic structure other than the mere 
general form is ever exhibited. Besides, like the Fuci, they were very 
partial to a sandy bottom, being comparatively rare in limestones, 
whilst the corals and radiated animals are chiefly found in the latter. 
Fucoides Harlani is extremely abundant in the red shales of Medina 
and Rochester, and in the equivalent sandstones and shales of Pennsyl- 
SiLURiAN System. 
Brachiopoda, 
Strophomena, (lower and medial 
strata.) 
Crustacea, 
Carboniferous System. 
Brachiopoda, 
Producta. 
Crustacea, 
Very rare, and the genera unde- 
termined. 
Polyparia? 
Amplexus. 
Isotelus, ^ 
Triarthrus, > (lower strata.) 
Ceraurus, ) 
Polyparia, 
