618 [April 30. 



Science, and some which had hitherto never been found lower than 

 the carboniferous limestone. Of the genus Onchus, found by myself 

 in the Old Red Sandstone of Worcestershire (see Sil. System, pp. 

 589, 596.) two species are described by M. Agassiz as the Onchus 

 heterogyrus and O. sublcevis, and of the new genus Byssacanthus 

 to which the Onchus arcuatus of Bromyard in Herefordshire is 

 now referred, that author describes two Russian species, B. crenu- 

 •latus and B. Icevis. From the other placoids from this locality 

 Professor Agassiz determines the new genera Homacanthus, 

 Hoplacanthus, Odontocanthus (one of the latter having pre- 

 viously been called by him a Ctenoptychius), Narcodes, and Naulos. 



Among the Cestraciont family of Placoids he places two of the 

 Russian forms in the genus Ctenodus, no species of which has 

 hitherto been found lower than the Carboniferous system. Of 

 this genus, two new and very remarkable species occur in 

 the lower Devonian beds of Russia, and these have been named 

 Ctenodus Keyserlingii and C. Worthii, after Count Keyserling and 

 Dr. Worth, who discovered them. Among the family of Hybo- 

 donts, M. Agassiz has recognised the new genus Cladodus and 

 others of which I have yet received no account from him. 



As these ichthyolites are about to be pviblished in his " Mo- 

 nographie des Poissons du Systeme Devonien," and as their 

 relations will be also illustrated by a short description of the 

 •fossils in the work on Russia already referred to, it is un-^ 

 necessary that I should further advert to them. As a geologist, 

 however, I may be permitted to remark, that most of the genera 

 being entirely unknown in the overlying deposits (the underlying 

 Silurian deposits of Russia never having afforded the trace of a 

 fossil fish), and as the species of Ctenodus are entirely distinct 

 from any forms of that genus in the carboniferous limestone, these 

 fossils strongly sustain the view of M. Agassiz, that each great 

 formation or system has been the tomb of a peculiar group of 

 fishes. 



On the river Siass, my colleague. Count Keyserling, has ob- 

 served a succession from Lower Silurian to Devonian, analogous 

 to that which I have just described. The lower beds, consisting 

 of arenaceous limestone with small white concretions, and calca- 

 reous flags alternating with red and green marls, contain numerous 

 true lower Silurian types, including the Orthis calligramma Dalm, 

 O. plana, 0. injlexa, and O. extensa of Pander, together with 

 Asaphus expansus, Orthoceratites vaginatus, Favosites petropoli- 

 tanus, &c. These are at once, and quite conformably, overlaid by 

 other calcareous flags, and also with red and yellowish marls, in 

 which certain typical shells never yet found in the upper Silurian 

 rocks, but eminently characteristic of Devonian strata, are inter- 

 mixed with true Devonian ichthyolites. Among the shells are, Orthis 

 striatula Schlot. Terebratula Livonica Von Buch, Spirifer mu- 

 ralis nob., Orthoceras cochleatum, and Serpula omphalotes, whilst 

 the fishes belong to the genus Dendrodus and the family of Coc- 

 costeini. This union in the vei'y same beds of the ichthyolites 



