604 rUV. TNVERTERRATA 



forms which agree with it in the position of the sipluincle ami (he 

 vshape of the septum. They re:icli their maxiiiunn clevelojiment in the 

 early I'alaeo/.oic, where tlie dominant forms have straiglit shells like 

 Orihoccras and Actinoceras, which were sometimes as much as 8 feet 

 long. It is diflicult to suppose that shelled animals of this size were 

 anythinjr other than sedentary organisms. There is a tendency for 

 the shell to become coiled in odier forms, exhibiting itself first in 



Fijr. 412. 



B 



T^ijT- 413- 



Fir. 412. Nauti/us nuicrowf^liolus ;ulhon"n)^ to the suhstratviin by moans of its 

 tontaclos in a vertical position. It usually lies horizontally. After Willey. 

 The shell shows alternate li^ht and dark hands which resemble "ripple- 

 marking", f//"- dorsal muscular attachments of the funnel ; c. eye ; hd. hood ; 

 mf. mantle; o. /(•//. ophthalmic tentacles. 



I'^i^. 413. A, Phylhiccrds fictcrof^ltyUiim, from the Lias: a part of the shell has 

 been removeil to expose the sutures, x ,|. H, Suture line of Phyll<ncras lictcro- 

 l^hylhtni, from the Lias: the arrow indicates the position of the siphuncle 

 and points towards the aperture of the shell. I'rom Woodward. Natural size. 



slightly curved forms like Cyrtoceras, then in loosely coiled forms like 

 (jyroCcras and finally in the closely coiled Ndutilus. There is also the 

 reverse tendency, ami in Lituilcs the young shell is closely coiled but 

 in adult life it straightens out completely. 



The ammonoids appeared first of all in the middle of the Palaeozoic 

 but reached their zenith in the Mesozoic. From the beginning of the 

 'IVias onward new families, genera ;md sj^ecies are ceaselessly evolved. 

 These are differentiated by the sliajie and sculj^turc of the shell 



