AMMONOIDEA. 37 



A. macrooephalus (15, 26, 28, 44, 4G, Earpoceras ptychophoedm (40). 



48, 59). A. subradiatus (23, 39). 



A. modiolaris (42). A. terebratus (5). 



By A. balceridB is not intended the species of Sowerby, which will be found 

 rather under sub-baherise. 



A. brocchii, A. humphriesianus, A.jurejisis, and A. subradiatus, are names assigned 

 by Professor Buckman, but never confirmed. The fossils to which they were 

 given were probably from the Kelloway Rock or from some underlying formation. 



A. bullatus appears to have been entered from a specimen which has been 

 squeezed transversely, and is not reliable as to its matrix, and in any ease cannol 

 be made out. 



.4. modiolaris is, no doubt, a fossil from the Oxford Clay of Stilton, though I 

 have not seen the specimen, nor any other from the Cornbrash. 



A. subradiatus, as recorded by Phillips, I have not seen, and expect it came 

 out of a lower bed, or is possibly an ally of Clydoniceras discus. 



A. terebratus is a synonym for a smooth type which the Macrocephalites are apt 

 to take on in the adult age. 



Besides these corrections there are the various generic names to which the 

 various species must now be assigned, as to which some introduction is called for. 



From time to time one after another of the features of an Ammonite have 

 been selected to form the foundation of their classification, and as one after 

 another comes to be studied the importance of the particular feature made the 

 subject for study impresses itself on the mind and becomes the basis of classifica- 

 tion. At one time it is the shape, at another the sutures, at a third the radial 

 lines, and so on. In my view, all these are of a partial character, only applicable 

 to certain minor groups. A general classification of the Cephalopoda, as of any 

 other group, must embody the history of that group and consist of a series of 

 classifications for each epoch, like a series of transverse sections across an animal. 

 Thus when the Cephalopoda first appeared the first variation was the position and 

 function of the siphuncle. When Ammonoids commenced the siplmncles had 

 taken up a final peripheral position, and their function had ceased to be of 

 importance. Nevertheless, there was still a division, into those the necks of whose 

 siplmncles were as before directed backwards and their sutures primarily lobed, 

 and those the necks of whose siplmncles were directed forwards and their sutures 

 provided with additional secondary lobes, forming the Retrosiphonata and Prosi- 

 phonata. These last, again, at a later date, were divided into Latisellati, with broad 

 saddles in 3^outh, and only the lobes secondarily cut, and the Angustisellati, with 

 narrow saddles in youth and indefinitely variable septa. Then among the latter 

 the septa became the seat of variation and had no common type, but their shell- 

 forms remained comparatively simple, until ultimately, from the Jurassic times 

 onwards, the sutures settled down into a fairly fixed type, to which the terms 



