Mr. W. King on the Tetrabranchiate Cephalopoda. 275 



to the dorsal or to the ventral aspect of the shell. Hence we have 

 no difficulty in graduating the Discus with its simple plates and 

 oscillating siphonal sheath, either into the Goniatite or the Cly- 

 menia. 



From the close-coiled Discus to the straight Orthocerus, our 

 passage is with as little difficulty effected through the genera 

 Gyrocerus and Cyrthocerus. 



Thus the Silurian period has furnished us with various cepha- 

 lopodous forms, which, notwithstanding their dissimilarity, may 

 be linked together in perfect harmony. We shall now endeavour 

 to connect them with some of the same class which have since 

 sustained their part throughout the various organic revolutions 

 of our globe. 



In passing from the Silurian to the Carboniferous period, most 

 of the forms which have been mentioned accompany us ; they 

 deviate however more and more from their original types as we 

 leave in the distance our starting-point : thus most of the Carbo- 

 niferous Goniatites differ from the simple-lobed species of the 

 Silurian rocks in having the edge of their plates more compli- 

 cated ; and several of the Carboniferous Nautiluses are distin- 

 guishable from their Silurian prototypes, the Discuses, by their 

 possessing the true conventional characters of the genus in which 

 they have been placed. 



The Cephalopods with a ventral siphon, as the beautiful Cly- 

 menias, do not appear to have undergone any amount of change ; 

 nor do they seem to have long survived the epoch of their crea- 

 tion. It is otherwise, however, with those which were furnished 

 with a dorsal siphon : they made their first appearance during the 

 Silurian epoch ; thence we are able to trace them through the 

 whole series of stratified deposits to the Cretaceous* sera, — thus 

 continuing throughout two immense geological periods, the pri- 

 mary and secondary. During this range, the edge of their plates 

 underwent a variety of modifications : in the first instance, the 

 plates had simple lobes, such as are displayed in the Silurian 

 Goniatites ; the crown of the dorsal and lateral posterior lobes, 

 however, became divided or digitated in the Carboniferous spe- 

 cies f; afterwards, for example, in the Triassic period, the divisions 



* The palaeontologist, who is of opinion that we are now acquainted ivith 

 the fossils of every geological period, may be disposed to question this asser- 

 tion, and to dwell upon the absence of these shells in the Permian rocks. 



f The simple dorsal lobe of the early Goniatites is divided in most of the 

 Carboniferous species, according to Vicomte d'Archiac and M. de Verneuil : 

 the posterior lateral lobes which verge on the dorsal aspect of the shell 

 are divided in G. mixolobus, G. cyclobus and G. Looneyi ; and in a species 



( ) from the western side of the Oural Mountains, specimens of which 



are in the Russian collection of the Newcastle Museum, the crown of all the 

 lateral lobes is divided. 



